STR U C TU R A L AND L U M IN E SC E N C E P R O P E R T IE S O F M A G N ESIU M
STR O N T IU M M E T A PH O S P H A T E D O PED W IT H E U R O PIU M AND
D Y SPR O SIU M IO NS
DAYANG N U R FA ZLIA N A B IN TI ABDUL H A L IM
U N IV E R SIT I T E K N O L O G I M ALAY SIA
STRUCTURAL AND LUMINESCENCE PROPERTIES OF MAGNESIUM
STRONTIUM METAPHOSPHATE DOPED WITH EUROPIUM AND
DYSPROSIUM IONS
DAYANG N UR FAZLIANA BINTI ABDUL HALIM
A thesis submitted in fulfillment o f the
requirements for the award o f the degree of
Master o f Science (Physics)
Faculty o f Science
Universiti Teknologi Malaysia
FEBRUARY 2011
iii
To m y Abah and M ama who have given me much more
than these few words can say.
For m y siblings and fiance, thanks for the
patience, love and prayers.
iv
ACKNOW LEDGEM ENT
Alhamdulillah. Thanks to Allah SWT, whom with His willing giving me
the opportunity to complete this master study. I am heartily thankful to my supervisor,
Prof Dr. Rosli Bin Hussin, whose encouragement, supervision and support from the
preliminary to the concluding level enabled me to develop an understanding o f the
subject. To all physics lecturers (UTM), thank you for the encouragement and immense
knowledge you gave all along.
I would also like to acknowledge the M inistry o f Science, Technology and
Innovation (MOSTI) for the financial support and kindly good encouragement in the
course o f m y study. M y sincere thanks and appreciation extend to the laboratory
assistants o f physics department who had helped me in many ways.
Deepest thanks and appreciation goes to my parents, family, m y fiance and all
my colleagues. I have been blessed with a friendly and cheerful group o f phosphor team
members. Lastly, I offer m y regards and blessings to all o f those who supported me in
any respect during the completion o f this study.
v
A B STR A C T
A series o f samples based on xM gO-(50-x)SrO-50P2Os (with 0 < x < 5 0 mol %)
were prepared using solid state reaction method and sintered at 900°C. The crystalline
phase o f powder samples were characterized using X-ray diffraction (XRD) while the
assignments o f the vibration modes were determined using Fourier Transform Infrared
(IR) and Raman spectroscopies. The XRD analysis indicated that the prepared samples
were polycrystalline phase o f Sr(PO 3)2, SrMgP 2O 7, Mg 2P 4O 12 and M g 2P 2O 7. The
optimum sintering temperature o f the prepared samples is 900oC. The Infrared and
Raman studies showed that the magnesium strontium metaphosphate system consists of
a main network o f Q2, Q 1 and Q° tetrahedral units. There were four peaks observed in
the both spectra which are P=O group (1320 cm-1), PO 2 group (1200 - 1170 cm-1), PO 3
and PO 4 groups (1160 - 950 cm-1) and P-O-P group (950 - 704 cm-1). This study
illustrated that SrO and MgO act as modifiers and also improved chemical and physical
stability o f the phosphate material. The dopant ions (Eu3+ and Dy3+) as studied using
XRD, IR and Raman spectra showed that small quantities o f europium and dysprosium
ions does not affect the local structure o f magnesium strontium metaphosphate network.
The luminescence property o f the Eu3+ and Dy3+ as dopants in magnesium strontium
metaphosphate was studied using photoluminescence spectroscopy. The emission peaks
for Eu3+ doped sample were located at 568 nm, 582 nm, 605 nm, 642 nm and 689 nm,
due to the 5D 0 —^ 7Fj (j = 0, 1, 2, 3, 4) transition. Meanwhile for Dy3+ doped sample, two
intense peaks appear at 477 nm and 564 nm are due to the
4F 9/2 — 6H 13/2 transitions. For
4F 9/2 — 6H 15/2 and
sample doped with Eu3+ and Dy3+, the intensity o f the
main peak increases. This study also showed that magnesium strontium metaphosphate
doped with Eu3+ and Dy3+ has better luminescence characteristic as compared to
strontium metaphosphate or magnesium metaphosphate. The results o f the study
suggested that magnesium strontium metaphosphate is a potential candidate for plasma
display applications.
vi
A BSTR A K
Satu siri sampel berasaskan sistem xMgO-(50-x)SrO-50P2O5 (dengan 0 < x < 5 0
mol %) disediakan melalui kaedah tindak balas keadaan pepejal dan disinter pada suhu
900oC. Kehabluran bagi sampel-sampel diciri menggunakan pembelauan sinar-X (XRD)
manakala sifat-sifat getaran ditentukan dengan menggunakan serapan infra merah (IR)
dan spektroskopi Raman. Analisis XRD menunjukkan sampel-sampel berada pada fasa
polihabluran Sr(PO3)2, SrMgP2O7, Mg2P4O 12 dan Mg2P2O7. Suhu optimum pensinteran
sampel ialah 900oC. Kajian serapan infra merah dan Raman menunjukkan sistem
magnesium strontium metafosfat mengandungi rangkaian utama Q2, Q 1 dan Q0 fosfat
tetrahedron. Terdapat empat puncak pada kedua-dua spektrum iaitu kumpulan P=O
(1320
cm-1), kumpulan
PO2 (1200
-
1170
cm-1), kumpulan
PO3 dan PO4
(1160 - 950 cm-1) dan kumpulan P-O-P (950 - 704 cm-1). Kajian menjelaskan
penambahan SrO dan MgO bertindak memantapkan kestabilan kim ia dan fizikal bahan
fosfat. Bahan pendop (Eu3+ and Dy3+) yang dikaji menggunakan spectrum XRD, infra
merah dan Raman menunjukkan kuantiti kecil ion europium dan disprosium tidak
menjejaskan rangkaian struktur magnesium strontium metafosfat. Pencirian luminesen
Eu3+ dan Dy3+ sebagai bahan pendop dalam magnesium strontium metafosfat diukur
menggunakan spektroskopi fotoluminesen. Puncak pemancaran bagi sampel didop
dengan Eu3+ ialah pada 568 nm, 582 nm, 605 nm, 642 nm dan 689 nm, disebabkan
5
7
3+
peralihan D0 — Fj (j= 0, 1, 2, 3, 4). Sementara itu untuk sampel didop Dy , dua
puncak pancaran utama dilihat pada 477 nm dan 564 nm yang disebabkan oleh peralihan
pada 4F9/2 — 6H 15/2 dan 4F9/2 — 6H 13/2. Untuk sampel yang didop dengan kedua - dua
ion iaitu Eu3+ dan Dy3+, keamatan puncak utama semakin meningkat. Kajian juga
menunjukkan magnesium strontium metafosfat yang didop dengan Eu3+ and Dy3+
mempunyai puncak pancaran yang lebih tinggi berbanding dengan strontium metafosfat
atau magnesium metafosfat. Keputusan kajian mencadangkan magnesium strontium
metafosfat sebagai salah satu calon yang berpotensi untuk aplikasi paparan plasma.
vii
TA B LE O F CO N TEN TS
CH APTER
1
T IT L E
PA G E
T IT L E PA G E
i
D E C LA R A TIO N
ii
D ED IC A T IO N
iii
ACKNOW LEDGEM ENTS
iv
A B STR A C T
v
A B STR A K
vi
TA BLE O F C O N TEN TS
vii
L IS T O F TA BLES
x
L IS T O F F IG U R E S
xi
L IS T O F SYM BOLS
xiv
L IS T O F A PPE N D IC ES
xv
IN T R O D U C TIO N
1.1
General Introduction
1
1.2
Statement o f Problem
3
1.3
A im so fth e S tu d y
4
1.4
Scope o f the Study
4
1.4.1
Sample Preparation
5
1.4.2
SampleCharacterization
5
1.5
Significantofthe Research
5
viii
2
L IT E R A T U R E R E V IE W
2.1
Introduction
7
2.2
Structural Characteristic
7
2.2.1
X-ray Diffraction
8
2.2.2
Fourier Transform Infrared Spectroscopy
9
2.2.3
Fourier Transform Raman Spectroscopy
11
2.3
Luminescence Characteristic
13
2.3.1
M echanism of Luminescence
14
2.3.2
T y p eso f Luminescence
16
2.3.3
Luminescence o fR areE arth Ions
18
2.3.4
E ffecto fD o p in g Io n so n Luminescence
20
Properties
2.4
3
4
Phosphor M aterial
23
2.4.1
PhosphateB asedPhosphor
24
2.4.2
Basic structure o f phosphate
24
2.4.3
Alkali Earth Phosphate
27
M ETHODOLOGY
3.1
Introduction
30
3.2
Sam plepreparation
31
3.3
Experimental Characterizations
34
3.3.1
X-ray Diffraction
34
3.3.2
FourierTransform InfraredSpectroscopy
35
3.3.3
FourierTransform R am anSpectroscopy
36
3.3.4
Photoluminescence Spectroscopy
37
R ESU LTS AND D ISCU SSIO N
4.1
Introduction
38
4.2
Structural Study
38
4.2.1
C rystallinePhaseA nalysis
39
4.2.2
Infrared Spectra Analysis
44
ix
4.2.3
4.3
Raman Spectra Analysis
54
Luminescence Study
59
4.3.1
Luminescence S p ectrao f Europium Ion
61
4.3.2
Luminescence S p ectrao f Dysprosium Ion
65
4.3.3
Luminescence S p ectraofE uropium and
68
Dysprosium Ions
5
C O N C LU SIO N S AND R EC O M M EN D A TIO N S
5.1
Summary
71
5.2
Future Study
73
R E FE R E N C E S
74
A PPE N D IC ES
82
X
L IS T O F TA BLES
TA BLE NO.
2.1
T IT L E
ElectronicC onfigurationsofTrivalent Rare-Earth
PA G E
20
Ions in the Ground State (Kano, 2006)
2.2
The types o f phosphates and their description (Brow, 2000)
27
3.1
The composition o f phosphor samples
31
4.1
C ry stallographicdataandlatticeparam eterofcrystal
41
phase observed in this study
4.2
Peak frequencies (cm-1) observed in the IR spectra o f
49
the xM g0-(50-x)Sr0-50P20 5 (with 0 < x < 5 0 mol %)
4.3
Peak frequencies (cm-1) observed in the Raman spectra
o f the xM g0-(50-x)Sr0-50P20 5 (with 0 < x < 5 0 mol %)
58
XI
L IS T O F FIG U R E S
FIG U R E NO.
T IT L E
PA G E
2.1
P rincipleofX -ray Diffraction
9
2.2
Schematic diagram o f a Fourier transform
11
Infrared spectrometer
2.3
S chem aticdiagram of a R am anspectroscopy
13
2.4
(a)E x citatio n an d (b ) em issio n sp ectrao f
21
Eu 3+:Li2T i0 3 systems as reported by Kumar and
B uddhudu(2009)
2.5
Luminescence intensities o f
22
Ca 5La 5(Si 0 4 ) 3(P 0 4 )3 0 2 : Dy3+
(Yang and Huang, 2007)
2.6
Phosphor devices in various fields o f applications
25
(Shinoya, 2001)
2.7
P-tetrahedral sites that can exist in phosphate host
26
(Hudgens etal., 1998)
2.8
XRD patterns for calcium pyrophosphate at
29
different sintering temperature (Doat et al., 2005)
3.1
The phosphor sample (a) after the sintering
32
process (b) under ultraviolet lamp
3.2
F low ch arto fsam p lep rep aratio n
33
3.3
X-ray Diffractometer (Siem ensDiffractom eter
34
Xll
D5000) at Faculty ofM echanlcal Engineering,
Universiti Teknologi Malaysia, Skudal
3.4
FTIR instrument at Chemistry Department,
35
Universiti Teknologi Malaysia, Skudai
3.5
Raman instrument at Universiti Teknologi
36
Malaysia, Skudai
3.6
Equipment used for photoluminescence
37
spectroscopy at School ofPhysics,
Universiti Sains Malaysia
4.1
XRD patterns at different sintering temperatures
40
4.2 (a)
X-ray diffraction patterns o f
42
xMgO-(50-x)SrO-50P20 5 with composition in
the range 0 < x < 2 5 mol %
4.2 (b)
X-ray diffraction patterns o f
43
xM g0-(50-x) Sr0-50P20 5 with composition
in the range 30 < x < 5 0 mol %
4.3
XRD patterns o fM g 0 S r0 P 20 5 (a) undoped
45
(b) doped Eu3+ (c) doped Dy3+ (d) doped Eu3+
and Dy3+
4.4 (a)
FT-Infrared spectra o f xM g0-(50-x)Sr0-50P20 5
46
powder samples (with 0 < x < 2 5 mol %)
4.4 (b)
FT-Infrared spectra o f xM g0-(50-x)Sr0-50P20 5
47
powder samples (with 30 < x < 5 0 mol %)
4.5
FT-Infrared spectra o f xM g0-(50-x)Sr0-50P20 5
51
powder samples (with 0 < x < 5 0 mol %) at
higher frequencies
4.6
IR spectra o f 25M g0-25Sr0-50P 20 5 (a) undoped
52
(b) doped Eu3+ (c) doped Dy3+
(d) doped Eu3+ and Dy3+
4.7
The phosphate tetrahedron structure
53
4.8
Schematic structure o f magnesium replacement
54
Xlll
in part o f strontium lon for magnesium
strontium phosphate
4.9 (a)
Raman spectra o f xMgO-(50-x)SrO-50P20 5
56
powder samples (with 0 < x < 2 5 mol %)
4.9 (b)
Raman spectra o f xM g0-(50-x)Sr0-50P20 5
57
powder samples (with 30 < x < 5 0 mol %)
4.10
Raman spectra o f2 5 M g 0 -2 5 S r0 -5 0 P 20 5
60
(a) undoped (b) doped Eu3+ (c) doped Dy3+
(d) doped Eu3+ and Dy3+
4.11
The luminescence spectra o f
62
25M g0-25Sr0-50P 20 5 for (a) undoped and
(b) doped with Eu3+
4.12
The energy level for Eu3+ in
64
25M g0-25Sr0-50P205
4.13
The luminescence spectra o f
66
25Sr0-25M g0-50P 20 5 for (a) undoped and
(b) doped Dy3+
4.14
The energy level o f Dy3+ in
67
25M g0-25Sr0-50P205
4.15
The luminescence spectra o f Eu3+ and Dy3+
69
doped in (a) 50Sr0-50P20 7 (b)50M g0-50P20 7
(c) 25M g0-25Sr0-50P205
4.16
The energy level model o f Eu3+ and Dy3+ in
magnesium strontium metaphosphate
70
xiv
L IS T O F SYM BOLS
3
-
Deformation
vas
-
Asymmetric Stretching
vs
-
Symmetric Stretching
E
-
Energy
X
-
Wavelength
S
-
Spin angular momentum
L
-
0 rbital angular momentum
J
-
Totalangularm om entum
I
-
Azimuthal quantum number
c
-
Speed o f Light
v
-
Frequency o f light
d
-
Distance
0
-
Angle
h
-
Planck constant
Q
-
T etrahedralofphosphate link
xv
L IS T O F A PPE N D IC ES
A PPE N D IX
T IT L E
PA G E
A
The Samples Calculation
82
B
Publications
85
CH APTER 1
IN T R O D U C TIO N
1.1
G eneral In tro d u ctio n
Luminescence is related to the basic science as well as to the applied science.
The word luminescence was first used by Eilhardt Wiedemann, a German physicist, in
1888 which means light (Williams, 1966). Nowadays, luminescence is observed with all
phases o f matter including gases, liquids and solids both organic and inorganic. The
commercial importance o f luminescence is ubiquitous, being manifest in displays,
lamps, biochemistry, environmental sciences, pharmaceuticals and others fields. In
nature, luminescence exists in fireflies, insects, fishes, mushrooms and luminescent
bacteria.
Luminescence can be defined as the generation o f light in excess o f thermal
radiation. Excitation o f the luminescence substance is prerequisite to luminescent
emission. There are several types o f luminescence depending on their source of
2
excitation which are photoluminescence, cathodoluminescence, electroluminescence,
triboluminescence and chemiluminescence.
Materials that can generate luminescence are called phosphors. Commercial
phosphors are mostly inorganic compounds prepared as powders, glasses or thin films
(Cees and Alok, 2006). Phosphor materials have attracted much attention because o f
their application to flat panel displays (FPD), including field-emission displays (FED).
The long after glow phosphor is a special property in luminescence field o f research. It
can maintain the phosphorescence for several hours at room temperature. In the early
time, zinc sulphide doped cobalt and copper (ZnS: Cu, Co) was considered the main
phosphorescent materials. However, it has a short phosphorescence time and not stable
enough during its application (Bol et al., 2002). Thus new hosts are getting much
attention in order to improve the long after glow properties and their stability.
Since its invention in the 19th century, the long lasting phosphor has been greatly
improved. Phosphate based is particularly an attractive host because it can accommodate
large concentrations o f active ions without losing the useful properties o f the material. It
also has several special properties such as large thermal expansion coefficients, low
melting temperatures, solubility and stability at higher temperatures (Day et al., 1998).
As example, Ca 5La 5(Si 0 4)3(P 0 4)30 2:Dy3+ is considered as an excellent commercial
white light long lasting phosphor that has been widely applied in the development of
fluorescent lamps, cathode ray tubes, field emission displays and plasm a display panels
(Yang and Huang, 2007).
Alkaline earth phosphates has attracted research interests in the field of
photoluminescence since they are suitable hosts with high chemical stability, offers
better homogeneity and lowers sintering temperature and also can produce plenty of
crystal field environments imposed on emission centers. Alkali earth phosphates
3
constitute a wide family, which, to our knowledge, has been little explored, although
their biocompatibility is well established (Liu et al., 2007). The developments of
phosphate based materials for a variety o f technological applications, from rare-earth
ion hosts for solid state lasers to low temperature sealing glasses, have led to renewed
interest in understanding the structures o f these unusual materials.
0 ver the past few decades, much attention has been devoted towards trivalent
lanthanide based materials for the development o f optical devices such as solid-state
lasers, fiber amplifiers and infrared to visible up-converters (Walrand and Binnemans,
1998). For example, Ferhi et al. (2009) reported that L aP 0 4:Eu3+ has been identified as
another good phosphate phosphor which displays an intense red emission under an
ultraviolet (UV) source. Up to now, rare earth ion doped luminescence materials
become an interesting topic in the field o f luminescence material.
1.4
S tatem ent of Problem
Among the borate, silicate and aluminate hosts materials, phosphate system
seems to be the most appropriate for investigations due its lower melting temperature,
higher thermal expansion coefficient and easy o f fabrication techniques. However the
low chemical resistance and moisture degradation o f phosphate systems poses many
restrictions on their commercial exploitation and usefulness. The studies by Shaw and
Shelby (1988) showed various oxides such as S n 0 , P b 0 , Z n 0 and Fe 20 3 improve
dramatically the chemical durability. However, the addition o f alkaline earth, S r0 and
M g 0 into phosphate networks is limited in structural study. Thus, this study should be
useful in determining the structural properties o f magnesium strontium metaphosphate
system.
4
Even though many new long lasting phosphorescent materials are developed, the
luminescence phenomenon activated by lanthanide elements remains to be explored.
Besides that, most o f the luminescence emissions in aluminate and silicate host material
are in the ultraviolet and infrared region.
Thus, in this study, magnesium strontium
metaphosphate doped with Eu3+ and Dy3+ ions were studied in order to determine their
luminescence properties.
1.5
Aims o f the Study
The aims o f this study are as follows:
1. To prepare and determine the structure and crystalline phases o f magnesium
strontium metaphosphate.
2. To determine the influence o f the modifier concentration, M g 0 and S r0 to the
phosphate network.
3. To determine the luminescence properties o f europium and dysprosium ions doped
in magnesium strontium metaphosphate material.
1.4
Scope o f the Study
Researches based on phosphate material are too wide. Thus, sample preparation
and its characterization are controlled in this study.
5
1.4.1
Sam ple P rep aratio n
The host material based on metaphosphate was prepared using solid state
reaction method. M agnesium oxide and strontium oxide were used as modifiers in order
to reduce the hygroscopic properties. In addition, Eu3+ and Dy3+ were chosen to be the
dopant ions for this study in order to study the effect o f dopants on the luminescence
properties.
1.4.2
Sam ple C h aracterizatio n
Different types o f measurements were used in this study. The X-ray diffraction
(XRD) was used to determine the phase o f the obtained samples, while FT -Infrared and
FT-Raman spectroscopy were used to determine the structural features o f the host
material. Luminescence spectra were obtained from photoluminescence spectroscopy.
1.5
Significant o f the R esearch
We expect to obtain samples that are chemically and physically stable which can
be easily prepared at lower melting temperature and lower cost compared to other host
matrix material such as silicate, tellurite and aluminate. In addition, it is hoped that the
doping ions; Eu3+ and Dy3+ can enhance the luminescence characteristic in magnesium
strontium metaphosphate materials and glow in the visible region. These properties
6
contribute a lot o f knowledge and a good reference for further research in developing
the luminescence technology for applications o f phosphor in electronics display and
luminous painting.
CH APTER 2
L IT E R A T U R E R E V IE W
3.1
In tro d u ctio n
This chapter describes on the background knowledge o f structural study and
luminescence o f phosphor material. In addition, the role o f doping ion, E u3+ and Dy3+
and theory related to the material characterization are convoluted for a better
understanding.
2.4
S tru c tu ra l C h aracteristic
It is necessary to understand the general information and importance o f structural
characteristic in material science. Structural study basically analyses using X-ray
8
diffraction, Fourier transform infrared and Raman spectroscopies. The theoretical
aspects are discussed as follows.
2.2.1
X -ray D iffraction
X-ray diffraction (XRD) uses the pattern produced by the diffraction o f x-rays
through the lattice o f atoms in a material to reveal the natural (symmetric) structure of
that lattice, which usually leads to an understanding o f the structure o f the material. The
lattice constants, n o f compounds are determined using Bragg’s law.
n l = 2 d sin 6
(2.1)
The X-ray physically interact with the electrons that surround the atoms. As
shown by schematic diagram in Figure 2.1, the X-rays scattered from a crystalline solid
can constructively interfere, producing a diffraction pattern. This technique works
because in a crystalline material the inter-atomic distances are o f the order o f the
wavelength o f X-ray radiation (10"8 cm). As a result, X-ray radiation is diffracted by the
electron clouds in the crystal structure. This gives us diffraction maxima which are
dependent on the type o f atoms, and their spacing and distribution. The X -ray diffraction
technique, therefore, is not a technique that produces images. However, the electron
density can be reconstructed using the diffraction pattern which is obtained from a
periodic assembly o f molecules in the crystal. Using this, a model can prelim inarily be
created. Then, calculated diffractions can be compared with observational data, to
finally produce an accurate unambiguous 3D physical structure o f the material.
9
n^ = 2dhkl sin0
F igure 2.1: Principle o f X-ray Diffraction.
2.2.2
F o u rier T ran sfo rm In fra re d Spectroscopy
Fourier transform infrared spectroscopy (FTIR) is well-known as an important
analytical tool in the study o f the structure o f both organic and inorganic materials as it
enables the arrangement o f the molecules, the bond type, morphological information and
the force fields in the material to be investigated. Infra-red spectroscopy is the subset of
spectroscopy that deals with the infrared region o f the electromagnetic spectrum. It
covers a range o f techniques, the most common being a form o f absorption
spectroscopy. The advantages o f infrared spectroscopy include wide applicability, non­
destructiveness, measurement under ambient atmosphere and the capability o f providing
detailed structural information
10
The infrared spectrum can be divided into two regions, one called the functional
group region and the other the fingerprint region. The functional group region is
generally considered to range from 4000 to 1500 cm-1 and all frequencies below
1500 cm-1 are considered characteristic o f the fingerprint region. The fingerprint region
involves molecular vibrations, usually bending and vibration motions that are
characteristic o f the entire molecule or large fragments o f the molecule. Thus these are
used for identification. The functional group region tends to include motions, generally
stretching vibrations, which are more localised and characteristic o f the typical
functional groups, found in organic molecules. While these bands are not very useful in
confirming identity, they do provide some very useful information about the nature of
the components that make up the molecule.
A schematic diagram o f a Fourier transform infrared spectrometer is shown in
Figure 2.2. The apparatus derives from the classical attempt by Michelson to measure
the ‘ether w ind’ by determining the velocity o f light in two perpendicular directions. An
FTIR spectrometer consists in principle o f an infrared source, an interferometer, the
sample, and the infrared detector. A parallel beam o f radiation is directed from the
source to the interferometer, consisting o f a beam splitter which is a plate o f suitably
transparent material coated so as to reflect only 50 percent o f the radiation falling on it.
Therefore, the spectrum is not directly measured but its interferogram where the IR
intensity reached the detector as a function o f the mirror position. The spectrum is
subsequently obtained by Fourier transformation o f the interferogram from the time
domain into the frequency domain.
11
1 Fixed mirror
A
k
Moving mirror
Beamsplitter
From IR source
/ /
*
*
*
n
r
<
----------►
| Sample
r
To de tector
F igure 2.2: Schematic diagram o f a Fourier transform Infrared spectrometer.
2.2.3
F o u rier T ran sfo rm R am an Spectroscopy
Raman spectroscopy is the measurement o f the wavelength and intensity o f
inelastically scattered light from molecules which usually used to study vibrational,
rotational, and other low-frequency modes in a system. It relies on Raman scattering of
monochromatic light, usually from a laser in the visible, near infrared, or near ultraviolet
range. The laser light interacts with phonons or other excitations in the system, resulting
in the energy o f the laser photons being shifted up or down. The shift in energy gives
information about the phonon modes in the system. Infrared spectroscopy yields similar,
but complementary information.
12
Light scattered from a molecule has several components which are Rayleigh
scatter and the Stokes and Anti-Stokes Raman scatter. In molecular systems, these
frequencies are principally in the ranges associated with rotational, vibrational and
electronic level transitions. The scattered radiation occurs over all directions and may
also have observable changes in its polarization along with its wavelength. The
scattering process without a change o f frequency is called Rayleigh scattering, and is the
same process described by Lord Rayleigh and which accounts for the blue color o f the
sky. A change in the frequency o f the light is called Raman scattering. Raman shifted
photons o f light can be either o f higher or lower energy, depending upon the vibrational
state o f the molecule.
For vibrational study, Raman technique has several advantages over infrared
spectroscopy. Firstly, the incident and scattered radiation are at ultraviolet and visible
frequencies, conventional optics and samples cell (crystal, glass, quartz) can be used, so
avoiding the problem o f atmospheric absorption and etc. the other reason is the beam
can be focused extremely fine (diameters as small as 0.1 nm are possible) thus very
small sample can be studied. This property combined with pulsed technique which can
give very short time resolutions, enables very small quantities o f transient species to be
studied. Lastly, liquid or moisture samples can be studied using Raman spectroscopy
since in Raman, water is a weak scatterer compared to infrared spectroscopy.
The working principle can be explained by the schematic diagram in Figure 2.3
below. The laser beam is passed through a cell, usually a narrow glass or quartz tube
filled with the sample. Light scattered sideways from the sample is collected by a lens
and passed into a grating monochromator similar to that used in a dispersed infrared
instrument.
The signal is measured by
a sensitive photomultiplier and after
amplification; it is usually processed by a computer which plots the Raman spectrum.
13
F igure 2.3: Schematic diagram o f a Raman spectroscopy.
3.4
Lum inescence C h aracteristic
Luminescence researches are associated with the dosimetric appliances, industry
and technology applications. In general, luminescence can be defined as the generation
o f light in excess o f thermal radiation. M an’s fascination with luminescence stems due
to their excellent characteristic emission bands ranging from ultraviolet to visible and to
infrared wavelength region.
The luminescence material is not new in our universe. For example, fireflies
have a bioluminescent organ in their abdomen and they used it to attract their mates.
Fireflies produce light by means o f chemical reaction that take place within their bodies.
14
They convert a compound known as luciferin from one form into another. Chemical
within the organ react with oxygen to produce light (Branham and Greenfield, 1996).
The terminology o f luminescence starts when an alchemist, Vincentinus
Casciarolo o f Bologna from Italy, found a heavy crystalline stone with a gloss at the
foot o f a volcano, and fired it in a charcoal oven intending to convert it to a noble metal.
Casciarolo obtained no metals but found that the sintered stone emitted red light in the
dark after exposure to sunlight. This stone was called the “Bolognian stone” and now
known as (B aS 0 4), with the fired product being BaS, which become a host for phosphor
materials.
Fifty years after the discovery o f the phosphorescent “Bolognian stone”, Zecchi
in 1652 advanced understanding o f the phenomenon by noting that the color o f the
phosphorescent light was the same after illumination by light o f different colors. Then
two hundred years later, Stokes clarified the nature o f fluorescent by showing that the
scattered and incident light differed in refrangibility (color). He noted that the
fluorescent light was usually less refrangible (longer wavelength) than the exciting light
which is known as Stokes’ Law (Williams, 1966).
2.3.1
M echanism of L u m inescence
The emission o f visible light requires excitation energies the minimum o f which
is given by Einstein’s law stating that the energy (E) is equal to Plank’s constant (h)
times the frequency o f light (v), or Planck’s constant times the velocity o f light (c) in a
15
vacuum divided by its wavelength (A,) as shown in equation (2.2) below (Blasse and
Grabmaier, 1994).
j-i j
hc
E = hv =
1
(2.2)
—
The
excitation
energy
is
transferred
to
the
electrons
responsible
for
luminescence, which jum p from their ground-state energy level to a level o f higher
energy. The energy levels that electrons can assume are specified by quantum
mechanical laws. The different excitation mechanisms considered below depend on
whether or not the excitation o f electrons occurs in single atoms, in single molecules, in
combinations o f molecules, or in a crystal. They are initiated by the means o f excitation
described above: impact o f accelerated particles such as electrons, positive ions, or
photons. Often, the excitation energies are considerably higher than those necessary to
lift electrons to a radiative level; for example, the luminescence produced by the
phosphor crystals in television screens is excited by cathode-ray electrons with average
energies o f 25,000 electron volts. Nevertheless, the colour o f the luminescent light is
nearly independent o f the energy o f the exciting particles, depending chiefly on the
excited-state energy level o f the crystal centres.
Electrons taking part in the luminescence process are the outermost electrons o f
atoms or molecules. When a solid is bombarded by photons or particles, the excitation
o f the centres can occur directly or by energy transfer. Light is directed onto a sample,
where it is absorbed and imparts excess energy into the material in a process called
photo-excitation. One way this excess energy can be dissipated by the sample is through
the emission oflight, or luminescence. In the case o f photo-excitation, this luminescence
is called photoluminescence. The photoluminescence spectrum provides the transition
energies, which can be used to determine electronic energy levels (Gfroerer, 2000).
16
Fluorescence and phosphorescence are two special aspects o f luminescence. The
phenomenon is fluorescence if emission takes place by one or more spontaneous
transitions. On the contrary; the emission occurs with the intervention o f a metastable
state followed by return to the excited stated due to addition o f energy is called
phosphorescence. Or in other word, fluorescence can be defined as emission during
excitation meanwhile phosphorescence is emission after excitation is removed.
2.3.2
T y p e so f Lum inescence
There are various o f luminescence phenomena observed in the nature or in
manmade articles. The emissive processes o f luminescence based on the type o f
excitation source (Vishwakarma et al. 2007).
Photoluminescence is a luminescence where the energy is supplied by
electromagnetic radiation. Photoluminescence is the emission produced by excitation
with the light photons. For example, one o f the high technology subjects called lasers is
a kind o f photoluminescence in which emission is coherent.
W hen excitation is done by electron beams generated at the electrical cathodes,
the emission produced is called cathodoluminescence. The screens o f cathode ray tubes
and television tubes glow by this kind o f emission. In cathode ray tubes, zinc and
cadmium sulphide phosphors are used.
Chemiluminescence is the luminescence where the energy is supplied by
chemical reactions. Those glow-in-the-dark plastic tubes sold in amusement parks are
17
examples o f chemiluminescence. Chemiluminescence is not a common accompaniment
in chemical reactions, because the amount o f energy released even in the exothermic
reactions is not sufficient to cause electronic excitation, which needs a couple o f eV
energy. Secondly not all chemical molecules are capable oflum inescence.
A large number o f inorganic and organic materials subjected to mechanical
stress
and
can
emit
light
is
called
triboluminescence.
It
also
named
as
mechnoluminescence. It has been observed in piezo electric crystals. The spectra of
triboluminescence light are similar to those o f photo-luminescence in many substances.
Bioluminescence is luminescence caused by chemical reactions in living things;
it is a form o f chemiluminescence. Fireflies, glow-worms, some bacteria and fungi and
many sea creatures, both near surface and at great depths are the striking examples of
luminescence in nature. The chemical reactions are the enzymic oxidations.
Electroluminescence
is
luminescence
caused
by
electric
current.
Cathodoluminescence is electroluminescence by electron beams; this is how television
pictures are formed. Other example o f electroluminescence is the auroras and lightning
flashes. This should not be mistaken for w hat occurs with the ordinary incandescent
electric lights, in which the electricity is used to produce heat, and it is the heat that in
turn produces light. There is another type o f electroluminescence known as injectionluminescence. In this electron are injected an external supply across a semiconductor pnjunction. On applying a DC voltage across thejunction, such that the electrons flow to
the p-region, luminescence is produced by the electron hole recombination in that
region. The light emitting diodes (LED), which are now commonly used as display
devices in many scientific instruments, are based on this principle.
18
Thermoluminescence is phosphorescence material triggered by temperature
above a certain point. In thermoluminescence, heat is not the primary source o f energy,
only the trigger for the release o f energy that originally came from another source. It
may be that all phosphorescence has a minimum temperature; but many have a
minimum triggering temperature below normal temperature and are not normally
thought o f as thermolunimescence.
Radioluminescence is luminescence caused by nuclear radiation. Older glow in- the dark clock dials often used paint with a radioactive material and a radio
luminescence material. There is a large variety o f inorganic minerals which shows
strong luminescence under excitation by X -ray and nuclear particles, although they
show no luminescence under UV excitation. Until a couple o f decade ago, the
luminescent dials o f watches and many instrument used to be phosphors to which
radioactive material were added to excite the luminescence.
2.3.3
Lum inescence of R are E a rth Ions
Rare earth ions have a long history in optical and magnetic applications. Among
these, luminescent devices using single crystals, powders, and glasses were particularly
important. Rare earths have important characteristic that distinguish them from other
optical active ions. They can emit and absorb over narrow wavelength ranges and the
quantum efficiencies tend to be higher except in aqueous solutions. In addition, the
wavelength o f the emission and absorption transitions are relatively insensitive to host
materials and the lifetime o f metastable states are long (Miniscalco, 2001).
19
All o f these properties resulted from the nature o f the states involved in these
processes and lead to the excellent performance o f rare earth ions in many optical
applications. In many applications, crystalline materials are preferred for reasons that
include higher peak cross section or better thermal conductivity, versatility o f glasses
and broader emission and absorption spectra.
There are fourteen rare earth elements and they lie between lanthanum ( 57La)
72
•
3+
3+
and Hafnium ( Hf). The lanthanides from Ce to Lu have one to fourteen 4 / electrons
added to their inner shell configuration. Ions with no 4 / electrons which are Sc3+, Y3+,
La3+, and Lu3+, have no electronic energy levels that can induce excitation and
3+
luminescence processes in or near the visible region. In contrast, the ions from Ce to
3+
Yb , which have partially filled 4 f orbital, have energy levels characteristic o f each ion
and show a variety o f luminescence properties around the visible region (Blasse and
Grabmaier, 1994). M any o f these ions can be used as luminescent ions in phosphors,
3+
3+
3+
31
mostly by replacing Y , Gd , La , and Lu in various compound crystals.
The azimuthal quantum number (l) o f 4 / orbitals is 3, which gives 2l + 1 = 7
orbital state and allows 14 electrons to stay. In the non-excited state the electrons are
distributed in such a way that it has the maximum combined spin angular momentum
(S). According to H und’s rule, the spin angular momentum S is added to the orbital
angular momentum L to give the total angular momentum J. For the lowest ground state,
J = L - S, when the number o f 4/ electrons is less or equal to 7, and J = L + S, when the
number o f 4/ electrons is larger than 7.
An electronic state is indicated by the notation 2S+1Lj , where L represents S, P, D,
F, G, H, I, K, L, M, ..., corresponding to L = 0, 1, 2, 3, 4, 5, 6 , 7, 8 , 9, ..., respectively
(Henderson and Imbusch, 1989). More accurately, an actual electronic state is expressed
20
as an intermediate coupling state, which can be described as a mixed state o f several
2S+1Lj states 2,4 combined by spin-orbit interaction.
T able 2.1: Electronic Configurations o f Trivalent Rare-Earth Ions in the Ground State
(Kano, 2006).
Atomic
Number
58
59
60
61
62
63
64
65
66
67
68
69
70
71
2.3.4
Ions
3+
Ce
3+
Pr
3+
Nd
3+
Pm
3+
Sm
Eu3+
3+
Gd
3+
Tb
3+
Dy
3+
Ho
Er3+
3+
Tm
3+
Yb
3+
Lu
4/electrons
S
£s
L
£i
J
£(L +S)
y2
3
5
5
4
9/2
4
5/2
1
3/2
2
5/2
3
7/2
3
5/2
5
3
2
3/2
i
6
6
1
1/2
0
0
0
7/2
3
5
6
6
6
8
5
3
6
0
0
15/2
15/2
7/2
E ffect of D oping Ions on Lum inescence P roperties
Among the Ln3+ ions used to optically activate the trivalent ion Eu3+ is mostly
3+
chosen due to the fact that Eu ions emit narrow band, almost monochromatic light and
3+
have long lifetime o f the optically activated states. Eu doped phosphors are commonly
used as red emitting materials for field emission technology and LEDs, which exhibit
higher luminescence efficiency compared with other luminous materials (Babu et al.,
2007).
21
W ada and Kojima (2007) reported that at present, Eu
3+
doped Y2O3, ScBO3,
YBO3 are used for red phosphors. Eu3+ doped ScBO3, YBO3 crystals have inversion
centre, posses higher transition possibility o f the 5D 0 ^
than the 5D 0 ^
7F 1 magnetic dipole transition
7F 2 electric dipole transition which leads to the advantages of
fluorescing an orange color.
Kumar and Buddhudu (2009) reported the luminescence by Eu3+ ions in titanate
system, as shown in Figure 2.4. The intense emission bands in the range o f 570 - 700
nm are assigned to the electronic transitions o f 5D0 ^ 7F 1>2,3,4 and the transitions such as
5D 0 ^
7F 2,4,6 transitions are electric dipole (ED) transitions. Among them, 5D 0 ^
7F 2 is
a hypersensitive transition and hence it demonstrates a bright red emission from
Eu3+ : Li2TiO3 system.
W avelength (nm)
F igure 2.4: (a) Excitation and (b) emission spectra o fE u 3+: Li2TiO3 systems as reported
by Kumar and Buddhudu (2009).
22
Dy
3+
is a good activator because o f two dominated band in the emission spectra
and its position depend strongly on the crystal field o f the host lattice used (Hussin et
al., 2010). O f the many rare earth ions, dysprosium ions are well known as an activator
dopant for many different inorganic lattices producing white light emission by suitably
adjusting the yellow and blue emission.
The luminescence lines o f D y3+ are in the 470 to 500 nm region due to the
4F 9/2 ^
6H 15/2 transition, and in the 570 to 600 nm region due to the 6F15/2 ^
6F1 1/2
transition. The color o f the luminescence is close to white. Yang and Huang (2007)
reported that Dy3+ ion doped in Ca 5La 5(SiO 4)3(PO 4) 3O 2 can emit white light with
Yellow / Blue =1. 1 (as shown in Figure 2.5).
450
500
550
600
650
700
W avelength (nm)
F igure 2.5: Luminescence intensities o f Ca 5La 5(SiO 4)3(PO 4)3O 2 : Dy
Huang, 2007).
3+
(Yang and
23
4.4
P h o sp h o r M aterial
Materials that generate luminescence are referred to as phosphors. Phosphor is a
solid which converts certain type o f energy into electromagnetic radiation over and
above thermal radiation. W hen a solid is heated to a temperature in excess until 600oC,
it emits red radiation and known as thermal radiation. The electromagnetic radiation
emitted by a phosphor material is usually in the visible range but can also be in the
ultraviolet and infrared regions.
Long histories about scientific phosphor are more than 100 years. A prototype of
the ZnS-type phosphors, an important class o f phosphors for television tubes, was first
prepared in 1866 by a young French chemist, Theodore Sidot, rather accidentally. It
seems that this marked the beginning o f scientific research and synthesis o f phosphors.
From the late 19th century to the early 20th century, Lenard and co-workers in Germany
performed active and extensive research on phosphors, and achieved impressive results.
They prepared various kinds o f phosphors based on alkaline earth chalcogenides
(sulfides and selenides) and zinc sulfide, and investigated the luminescence properties
(Leverenz, 1950).
Pohl and co-workers in Germany investigated alkali halide phosphors in detail in
the late 1920s and 1930s. They grew single-crystal phosphors and performed extensive
spectroscopic studies. Leverenz and co-workers at Radio Corporation o f America (U.S.)
also investigated many practical phosphors with the purpose o f obtaining materials with
desirable characteristics to be used in television tubes (Shionoya, 2001). The research o f
phosphor material continues until now due to higher demand in their applications.
24
Luminescence have some important applications in laser technology, ultrashort
time resolved spectroscopy, fiber optics, fluorescence probes, imaging techniques,
fluorescence microscopy. Figure 2.6 lists various kinds o f phosphor devices according
to the method used to excite the phosphor. It gives a summary o f phosphor devices by
the manner in which the phosphors are applied.
2.3.1
P hosphate Based P ho sp h o r
In the last 30 years, phosphate systems were investigated because o f their
interesting properties. The developments o f phosphate based materials for a variety of
technological applications, from rare earth ions host for solid state lasers to low
temperature sealing glasses, have led to renewed interest in understanding the structures
o f these unusual materials. In efforts to design systems with specific properties for such
application, the structure and properties o f alkali, alkali earth, transitional metal and rare
earth phosphate system are studied.
2.3.2
Basic stru c tu re of phosphate
Phosphate is well known as a salt o f phosphoric acid (H3PO4). The basic
building blocks o f crystalline and amorphous phosphates are the P-tetrahedra that result
from the formation o f sp3 hybrid orbitals by the P outer electrons (3s23p3). This
tetrahedral link through covalent bridging oxygens to form various phosphate anions.
25
F igure 2.6: Phosphor devices in various fields o f applications (Shinoya, 2001).
26
The tetrahedral are classified using the Q 1 terminology where ‘i ’ represents the
number o f bridging oxygen per tetrahedron (Brow, 2000). For an example Q 3
tetrahedron has three bridging oxygens and one non bridging oxygen. The possible
phosphate structures and their Q 1 designations are shown in Figure 2.7.
F igure 2.7: P-tetrahedral sites that can exist in phosphate host (Hudgens et al., 1998).
The networks o f phosphate systems can be classified by the oxygen-tophosphorus ratio, which sets the number o f tetrahedral linkages, through bridging
oxygens, between neighboring P-tetrahedra. Phosphate systems can be made with a
range o f structures, from a cross-linked network o f Q tetrahedra (vitreous P 2O5) to
polymer-like
metaphosphate
chains
of
Q 2 tetrahedra,
a
system
with
small
pyrophosphate, Q 1 and orthophosphate, Q 0 anions depending on the oxygen-tophosphorus ratio as set by the crystals compositions.
27
T able 2.2: The types o f phosphate and their description (Brow, 2000).
P H O SPH A TE
TY PES
TY PES
C O M P O S IT IO N
D ESC R IPT IO N S
(mol % )
q3+ q2
0
0
Ultraphosphate
•
Metaphosphate
50
Q2
O-P-O, PO 2, PO 4
in
phosphate
tetrahedral
•
Polyphosphate
q2+ q
1
34-49
P-O-P symmetry
stretching
vibration
Pyrophosphate
Orthophosphate
Q1
Q0
33
0-32
•
(POP)sym stretch.
•
P-O stretch.
•
(PO3)sym stretch
•
(PO4)sym stretch
(isolated)
2.4.3
A lkali E a rth P hosphate P h o sp h o r
Interest in alkaline earth phosphate system was stemmed from their high
transparency for ultraviolet (UV) light, when compared with silicate host. In the 1950s,
interest in amorphous alkali phosphates was stimulated by their use in a variety of
industrial applications, including sequestering agents for hard water treatments and
dispersants for clay processing and pigm ent manufacturing. More recently, phosphate
system were developed for a variety o f applications such as fluorescent lamp, light
28
emitting diode, flat display panel and etc. The properties that make phosphate system
candidates for so many different applications are related to their unique molecular-level
structures.
Among the phosphate system based materials, those containing calcium or
magnesium received a great deal o f attention due to their bioactivity or to unusual trends
between the variations o f composition versus physical properties respectively (Wang et
al., 2005). In both systems, the preparation o f glasses
/ crystals containing large
amounts o f alkali earths is important.
For example, calcium phosphate systems which contain a large amount o f calcium
(CaO / P 2O 5 > 2 molar ratio) have the ability to bond to natural bone, resulting in
materials suitable for implantation (Kitsugi et al., 1993). Although, natural bones are
multiphase materials and composite fabrication appears as a possibility to create such
materials, the preparation o f phosphate system with high calcium content, seems to be
one o f the best alternative approaches for bone bonding. The reason is that these systems
have chemical composition close to that o f the natural bone, can act as drug carriers and
their crystallization can be controlled in order to improve the microstructure o f the
bioactive implant.
On the other hand, binary system in the M gO -P 2O 5 exhibit an unusual
discontinuity in their composition / property behavior near the metaphosphate
composition (MgO / P 2O 5 ~ 1) (Wang et al., 2005). This anomalous behavior was
examined in relation to the M g-O coordination number (6 or 4) in phosphate structures
using various spectroscopic methods. According to these studies the Mg cations seems
to act as network modifiers in the xMgO(1 - x)P 2O 5 systems over the range o f x = 0.45 0.60 mol. Thus, the abrupt changes in the physical properties o f these systems were
29
attributed mainly to the changes in the intermediate-range order o f the phosphate
network (Suzuya et al., 1999).
Spectroscopic results from research done by Karakassides et al. (2004) showed
that the modification o f the phosphate network was higher for the Ca containing glasses
with respect to the Mg ones, at the same alkali earth content, due to the well defined Ca
properties as a modifying cation. The Mg cation exhibits higher glass forming abilities,
even at higher than the ortho composition.
Doat et al. (2005) reported in calcium pyrophosphate system, at 1250oC the X-ray
diffraction pattern grow in intensity and much more sharp as compared to the sample
calcined at 900oC. After heating at 900oC in air, two phases coexisted, identified as the P
calcium pyrophosphate form and EuPO4. Heating near 1250oC in air, during the P ^ a
transformation, europium ions substitute for a calcium ions in the a calcium
pyrophosphate structure as shown in Figure 2.8 below.
♦ Ca 3Eu(PO4)3
• EuPO 4
2 theta
F igure 2.8: XRD patterns for calcium pyrophosphate at different sintering temperature
(Doat et al., 2005).
CH APTER 3
M ETHODOLOGY
3.3
In tro d u ctio n
Sample preparation is an important stage in a research process. Depending on the
type o f sample, there are various ways to prepare it such as solid state reaction method,
sol gel technique, melt quench method or many other techniques. Sample preparation
may involve reaction with some chemical species, dissolution and others. Other
parameter such as sam ple’s composition and operating temperature also play an
important role in order to form a good sample. Thus this chapter deals with the tools and
techniques employed carried out in this work.
31
3.4
Sam ple p re p a ra tio n
The phosphor samples were prepared by solid state reaction. The batch mixture
(20 g) was prepared using raw materials o f reagent grade SrCO3, MgO and H 3PO 4 (85%
liquid). The samples were prepared based on the series o f xMgO-(50-x)SrO-50P2O5 with
composition in the range 0 < x < 5 0 mol % as shown in Table 3.1. From the analysis, the
selected sample with optimum composition was then doped with rare earths which are 1
mol % o f Eu 2O 3 and Dy 2O 3 respectively.
T able 3.1 : The composition o f phosphor samples.
Sam ple
M gO (mol % )
SrO (mol % )
P 2O 5 (mol % )
1
0
50
50
2
5
45
50
3
10
40
50
4
15
35
50
5
20
30
50
6
25
25
50
7
30
20
50
8
35
15
50
9
40
10
50
10
45
5
50
11
50
0
50
32
The starting materials with corresponding weights were mixed thoroughly in an
alumina crucible and placed in air in an electric furnace. A two-step sintering schedule
was employed to minimize volatilization losses o f low-melting starting materials as well
as to ensure homogeneous mixing o f the constituents. As the first step, the temperature
was set to 300oC with heating rate 25°C/min and maintained for 1 h to facilitate
evaporation o f water released by P 2O5 and CO2. For the second step, the temperature
was increased slowly to 900oC with heating rate o f 25oC/min and maintained for 4 h in
air. The product was cooled to room temperature. Then all the samples were ground and
sieved with 100 p,m sieve.
F igure 3.1: Phosphor sample (a) after the sintering process (b) under ultraviolet lamp.
33
F igure 3.2: Flow chart o f sample preparation.
34
3.3
E x p erim en tal C h aracterizatio n s
3.3.1
X -ray D iffraction
The structures o f the crystalline and amorphous prepared samples were analyzed
by means o f X-ray diffraction (XRD) instrument as shown in Figure 3.3. The samples
are in powders form. The XRD measurements were carried out with CuKa radiation
operating at 40 kV, 30 mA with Bragg-Brentano geometry at room temperature using
Siemens Diffractometer D5000, equipped with diffraction software analysis. Diffraction
patterns were collected in the 2-theta (20) range from 10 to 80o, in steps o f 0.05o and Is
counting time per step.
*
F igure 3.3: X-ray Diffractometer (Siemens Diffractometer D5000) at Faculty of
M echanical Engineering, Universiti Teknologi Malaysia, Skudai.
35
3.3.2
Fourier Transform Infrared Spectroscopy
Small quantities of sample powder were mixed and ground with relatively large
amount of KBr. KBr pellets, transparent to light were formed by pressing the mixture at
5 tons for a few minutes. The infrared spectra for the unannealed samples were recorded
immediately in a Perkin-Elmer 1710 Fourier transform infrared spectrometer over the
range 4000 cm - 1 - 400 cm -1 as shown in Figure 3.4.
Figure 3.4: FTIR instrument at Chemistry Department, Universiti Teknologi Malaysia,
Skudai.
36
3.3.3
Fourier Transform R am an Spectroscopy
The Raman spectra were measured with a Perkin-Elmer (Spectrum model
2000R NIR FT-Raman spectrometer) in the spectral range 100-4000 c m 1 as shown in
Figure 3.5. The laser power on the samples being used is 250 mW. The spectrometer
uses a ytterbium aluminum garnet crystal doped with triply-ionized neodymium
(Nd:YAG) as a lasing medium. The laser is software-controlled, diode-pumped running
at 810 nm, with power of 1600 mW or 750 mW TEM00 and typical working power
range from 100 mW to 1 W. The resultant Gaussian beam profile is vertically polarized
to better than 100:1. The amplitude stability is better than 0.1 rms and is controlled by
means of active optical feedback. The laser line width gives a spectral resolution of
better than 0.5 cm-1.
Figure 3.5: Raman instrument at Universiti Teknologi Malaysia, Skudai.
37
3.3.4
Photoluminescence Spectroscopy
Photoluminescence spectra were also recorded with a Jobin Yvon HR 800 UV
by HeCd laser excitation at wavelength of 325 nm as shown in Figure 3.6.
Figure 3.6: Equipment used for photoluminescence spectroscopy at School of Physics,
Universiti Sains Malaysia.
CHAPTER 4
RESULTS AND DISCUSSION
4.4
Introduction
The aim of this chapter is to determine the structure and crystalline phases of
magnesium strontium metaphosphate. In addition, the influence of MgO and SrO to the
phosphate network are also discussed. The luminescence studies of europium and
dysprosium ions doped in magnesium strontium metaphosphate material are also
presented in this chapter.
4.5
Structural Studies
Structural analysis is an important aspect in order to relate with the information
obtained from luminescence properties. X-ray diffraction (XRD) pattern leads to an
39
understanding of the structure of the material. Infrared and Raman spectra were used to
determine the role of modifier; magnesium oxide and strontium oxide in the phosphate
network.
4.2.1
Crystalline Phase Analysis
X-ray diffraction pattern is used to identify the crystalline phase whether the
samples are in the form of amorphous, polycrystalline or highly crystalline. Thus, this
section presents the crystalline phase of the prepared samples.
Figure 4.1 shows the XRD pattern of 25M g0-25Sr0-50P20 5 at different
temperature ranging from 700oC to 900oC. All the samples have shown a polycrystalline
phase at different temperature.
The samples sintered at 700oC and 800oC show a
broader diffraction pattern peaks compared to sample sintered at 900oC. The sharper
peaks at higher sintering temperature suggest that upon elevation of sintering
temperature, the crystallinity is continuously increasing. The highly crystalline phase
obtained at 900oC is similar to the results reported in pyrophosphate system. The XRD
patterns of calcium pyrophosphate showed an improvement in formation peaks
crystalinity when it is calcined at higher temperature compared to the lower temperature
(Doat et al. 2005).
All the XRD patterns are almost the same and can be indexed to the Sr(P03)2,
SrMgP2 0 7, Mg2 P4 0 12, and Mg2 P 2 0 7 as shown in Table 4.1. The phase of Mg 2 P 4 0
12
and
Sr(P0 3 ) 2 grow in term of intensity as the sintering temperature is increases. Mg 2 P 4 0 12
40
become a predominant phase in the sample followed by SrMgP 2 0 7 phase and Sr(P0 3 ) 2
and Mg2 P 2 0 7 as the minor phase formation.
For further analysis, 900°C has been chose as an optimum synthesis temperature
due the high crystalline phase obtained and well matched with the sample’s diffraction
pattern.
20
Figure 4.1: XRD patterns of 25M g0-25Sr0-50P20 5 at different sintering temperatures.
41
Table 4.1: Crystallographic data and lattice parameter of crystal phase observed in this
study.
Lattice Param eter
Crystal
Crystal
Space
Phase
Structure
G roup
a
b
c
Sr(P03)2
-
-
-
-
-
SrMgP2 0 7
Monoclinic
P 21 /n
5.309
8.299
1 2 .6 8
Mg2P4012
Monoclinic
C2 /c
11.756
8.285
9.917
Mg2 P 207
Monoclinic
B 21 /c
13.198
8.295
9.072
Phosphate usually have a hygroscopic behavior which makes them unsuitable for
practical application. Thus to overcome this problem, modifiers (Sr0 and M g0) have
been introduced. The influence of such modifiers on phosphate structure are presented
below.
Figure 4.2 (a) and Figure 4.2 (b) show the XRD patterns and the phase formation
of xMg0-(50-x)Sr0-50P20 5 with composition in the range 0 < x < 5 0 mol %. All the
samples are confirmed as polycrystalline phase due to the formation of sharp peaks
observed in the X-ray diffraction patterns. It is noted that Sr(P 0 3 ) 2 phase with JCPDS
file number 12-0366 is observed in the 50Sr0-50P20 5 binary system. After the addition
of magnesium oxide, there are three new phases observed which are SrMgP 2 0 7,
Mg2 P 4 0
12
and Mg 2 P 2 0 7. The appearance of these peaks indicates that the reaction of
ternary system has occurred in the prepared samples.
The intensity of Sr(P0 3 ) 2 peaks at 20 is 24o gradually decreased as the content
of magnesium oxide is increased. However, the formation of Mg 2 P 4 0 12 (JCPDS file
number: 70-1803) become the predominant phase accompanied with Mg 2 P2 0 7 (JCPDS
file number: 72-0019) as minor phase. The diffraction pattern of SrMgP2 0 7 with JCPDS
Intensity (a.u)
42
29
Figure 4.2 (a): X-ray diffraction patterns of xMg0-(50-x)Sr0-50P20 5 with composition
in the range 0 < x <25 mol %.
43
cd
Sr(P0 3 ) 2
b -----
SrMgP207
c
Mg2P4012
d
Mg2 P 2 0 7
c
x mol %
c
c
a — -•
c
d
bc
c
50
c
c
.ua.
45
b
ns
e
ac
aa
c a
bc
iJuAy^h
c
40
cb
cc
35
»
a a
a
c
30
c
d
10
20
30
40
50
60
70
20
Figure 4.2 (b): X-ray diffraction patterns of xM g0-(50-x)Sr0-50P20 5 with composition
in the range 30 < x < 50 mol %.
44
file number 49-1027 raises to its high intensity until the composition is 25M g0-25Sr050P20 5. This suggests that the reaction of a complete phase (SrMgP 2 0 7) of this ternary
system has occurred to a considerable extent and in agreement with the work done by
Wongmaneerung et al. (2006) in titanate systems.
The XRD patterns for 25M g0-25Sr0-50P20 5 doped with Eu3+ and Dy3+ are
shown in Figure 4.3. The undoped sample is also included in the spectra as a reference.
There is no identifiable difference seen in the pattern which indicate that a small amount
of rare earth ions does not affect the diffraction pattern in this study. This result is
consistent with Han et al. (2006) who claimed that there is no change in the peaks
3+
formation as Y V 0 4 phosphor is doped with Dy ion . Guifang et al. (2008) also
3+
reported that Eu gives no obvious changes in XRD peaks in YAl3 (B 0 3) 4 system.
4.2.2
Infrared Spectra Analysis
The vibration frequency of functional group and network structure are the
important information that can be provided by infrared spectroscopy. This section
presents the structure of phosphate network by IR probing.
The IR spectra of xMg0-(50-x)Sr0-50P20 5 (0 < x < 5 0 mol %) in the range of
400 to 1800 cm -1 are shown in Figure 4.4 (a) and Figure 4.4 (b). There are four main
region that can be recognized in this spectra which are 1320 cm -1 due to the P=0
vibration, the region around 1200 - 1170 cm -1 which characterize as non-bridging P 0 2
groups, the region near 1160 - 950 cm -1 are P 0 3 and P 0 4 groups and the region around
950 - 704 cm-1 known as P-0-P terminals.
Intensity (a.u)
45
29
Figure 4.3: XRD patterns of 25M g0-25Sr0-50P20 5 (a) undoped (b) doped Eu3+ (c)
doped Dy3+ (d) doped Eu3+ and Dy3+.
Intensity (a.u)
46
Wavenumbers, cm -1
Figure 4.4 (a): FT-Infrared spectra of xMg0-(50-x)Sr0-50P20 5 powder samples (with
0 < x <25 mol %).
Intensity (a.u)
47
Wavenumbers, cm "1
Figure 4.4 (b): FT-InfTared spectra of xMg0-(50-x)Sr0-50P20 5 powder samples (with
30 < x < 50 mol %.
48
All the assignments and the most important bands are given in Table 4.2 based
on the work done by other researchers in the phosphate network (Hussin et al. (2009),
Carta et al. (2007) and Shih et al. (1998). These spectra pattern (Figures 4.4 (a) and
Figure 4.4 (b)) show typical IR spectra observed in sodium lead cadmium
metaphosphate glasses (Hezzat et al., 2003) and sodium copper phosphate (Brow et al.,
1990). There is no significant difference in the line shape of the spectra among samples
containing 50 mol % P 2 0 5.
The stretching mode of double bonds in phosphate tetrahedral appears at
1341 cm-1. Several studies have shown that the band corresponding to the stretching
vibration of doubly bonded oxygen could be found in the frequency range 1230 - 1390
cm -1 (Hudgens et al. (1998), Hussin et al. (2009) and Baskaran et al. (2007)).
Strong bands near 1231 cm -1 and 1170 cm -1 are assigned to asymmetric and
symmetric stretching modes of two non-bridging oxygen atoms bonded to phosphorus
atom (P 0 2) in Q2 unit in the phosphate tetrahedral. These absorption bands shift to a
lower frequency as Sr0 is replaced by M g0. This is expected because the magnesium oxygen bonds are more covalent than the strontium-oxygen bond; thus the phosphorus oxygen bond linked to magnesium ion (P-0(-Mg)) are more ionic than P -0 (.. .Sr) bonds
(Shih et al., 1998). Brow et al. (1990) and Nelson and Tallant (1985) also reported that
stretching mode shifts in N aP 0 3 glass.
The absorption band near 801 cm -1 is due to the asymmetric stretching mode of
P-0-P
linkages while
symmetric
stretching P-0-P
groups
appear at around
704 - 746 cm-1. Both bands for P-0-P asymmetric and symmetric stretching shift to
higher frequency by the substitution o fM g 0 for Sr0. It can be explained by the increase
in covalent character o fP -0 -P bonds and indicates that the P-0-P bonds are
ble 4.2: Peak frequencies (cm'1) observed in the IR spectra of the xMgO-(5 0 -x)SrC)-5 0 P 2 0 5 (with 0 < x < 50 mol %).
S(P02)
5(P-0-P)
5(P 04)3"
Us (P-O-P)
Uas(P-O-P)
t>s(P 0 4)3"
uas(P 0 4)3" us(P20 7)4-
us(P 0 3)2"
uas(P 0 3)2-
us(P 0 2)
uas(P 0 2)
u
401
470
562
704, 737
801
939
1003
1074
1107
1162
1170
1
401
470
562
704, 740
802
939
1003
1074
1107
1163
1171
402
470
563
712,737
800
938
1003
1074
1109
1162
1174
402
471
562
712, 740
801
937
1004
1076
1110
1163
1173
403
471
562
712, 743
802
936
1003
1077
1107
1165
1176
404
471
562
712, 740
801
932
1005
1079
1108
1164
1181
403
472
563
712, 740
802
931
1004
1079
1107
1163
1184
403
472
563
712, 745
802
933
1003
1078
1106
1164
1186
403
473
563
718, 746
801
932
1004
1078
1107
1164
1188
402
472
562
718, 746
802
932
1005
1079
1105
1163
1188
403
473
562
718, 746
802
932
1005
1079
1105
1164
1189
1231,
1293
1231,
1294
1231,
1294
1232,
1294
1233,
1294
1231,
1295
1232,
1295
1231,
1296
1231,
1296
1231,
1296
1232,
1296
1
1
1
1
1
1
1
1
1
1
50
strengthened as Sr0 is replaced by M g0. Shih et al. (1998) also stated that the P-0-P
bonds are stronger by the substitution ofN a20 by C u0 in his study.
The presence of bands at 1231 cm -1 and 1293 cm -1 (uasP 0 2), 1170 cm -1 (usP 0 2),
801 cm-1 (uasP0P) and 704 cm-1, 737 cm-1 (usP0P) in this spectra implies that
metaphosphate chain is the principle building fragment of this structure. Ilieva et al.
(2001) reported that the structure of the us(P0P) bands is generally considered as the
most characteristic one in the spectra of metaphosphate since their occurrence is in a
range free from other vibrational frequencies. Furthermore the us(P0P) bands are very
sensitive to change in the configuration since the frequency is strongly dependent on the
P0P bond angle. The occurrence of the uas(P-0-P) at around 800 cm -1 and the analogous
vibration at 1000 cm-1 indicate a chain structure and ring structure respectively (Carta et
a l, 2007).
The infrared spectra at higher frequency are shown in Figure 4.5. It is observed
in the region 1600 cm -1 to 4000 cm-1. There is a broad IR band at around 3422 cm -1
corresponding to the stretching vibration of water molecules (vH2 0). The value is in
agreement with Carta et al. (2007) who reported that symmetric stretching of 0-H
groups for natrium calcium phosphate is around 3300 cm-1. The intensity of vH20 band
shows a minima intensity as the amount of Sr0 and M g0 is 25 mol% repectively. Thus
it is suggested that the introduction of Sr0 and M g0 as modifiers reduce the
hygroscopic properties of phosphate materials and consistent with the results obtained
by Sales et al. (1998) and Milankovic et al. (2001).0ne peak at 1644 cm -1 is also
observed in the spectra which belonged to the deformation of 0 -H groups. Assaaoudi et
al. (2005) also observed the SH20 vibration bands in the region 1685 cm-1. The
occurrence of these two bands (vH20 and JH 2 0 ) is probably due to water absorptions
during the pellet preparation with KBr.
Intensity (a.u)
51
Wavenumbers, cm-1
Figure 4.5: FT-Infrared spectra of xM g0-(50-x)Sr0-50P20 5 powder samples (with 0 <
x < 5 0 mol %) at higher frequencies.
52
The IR spectra for 25M g0-25Sr0-50P20s doped with Eu3+ and Dy3+ samples in
the frequency range 400 cm -1 to 4000 cm -1 is shown in Figure 4.6. The undoped sample
is also illustrated in the spectra. By comparing all of the spectra, it clearly shows no
changes in the finger print region. The vibrations that occur are almost the same with the
IR spectra reported in the previous section. Thus it can be concluded that small amounts
of rare earth doping will not affected the structure feature. This result is consistent with
Intensity a.u
the work done by Kumar and Buddhudu (2009).
Wavenumbers, cm -1
Figure 4.6: IR spectra of 25M g0-25Sr0-50P20s (a) undoped (b) doped Eu3+ (c) doped
Dy3+ (d) doped Eu3+ and Dy3+.
53
The basic structural feature of P 2 0 5 is a phosphate tetrahedron with three
bridging and one non-bridging (double - bonded) oxygens is illustrated in Figure 4.7.
The double-bonded oxygen arises from the pentavalency of phosphorus.
Figure 4.7: The phosphate tetrahedron structure.
When an alkali oxide is added to a phosphate sample, additional non-bridging
oxygen will be created at the expense of bridging oxygen. The structure as discovered
by Brow et al. (1988) is shown in equation (4.1). The non-bridging oxygens that are
created by the addition of alkali oxides to phosphate crystals are expected to be charge compensated by the alkali ions.
0
0
0 - P — 0I
0
li
+R 20
0
- P -
0
-
R
0
(4.1)
The structures of ternary magnesium strontium metaphosphate composition can
be represented as in Figure 4.8. The structure is a resonance chain structure (Shih et al.,
1998). When strontium ion is replaced by magnesium cation to form ternary magnesium
strontium metaphosphate crystals, it can be elucidated that P-0-M g bonds form and
replace P—0"... Sr2+, where as no P-0-P bond network is affected.
54
Figure 4.8: Schematic structure of magnesium replacement in part of strontium ion for
magnesium strontium metaphosphate.
4.2.3
R am an Spectra Analysis
Raman technique measures the wavelength and intensity of inelastically
scattered light from molecules which are usually used to study vibrational, rotational,
and other low-frequency modes in a system. Raman spectroscopy usually gives better
spectra compared to IR spectra. Thus Raman analysis gives more information about the
structure range order of magnesium strontium metaphosphate.
55
Raman spectra of xMg0-(50-x)Sr0-50P20s powder samples with composition in
the range 0 < x < 5 0 mol % are shown in Figures 4.9 (a) and Figure 4.9 (b). The
assignments of the peaks are given in Table 4.3, followed the guideline as reported by
Fang et al. (2001), Brow et al. (1990), and Nelson and Tallent (1985) in various glassy
and crystalline phosphates.
The Raman spectra of the investigated powder samples show five sharp peaks at
1320 cm-1, 1288 cm-1, 1169 cm-1, -664 cm-1 and -352 cm-1. The strong band near 1320
cm-1 is attributed to the stretching mode of P = 0 double bonds in phosphate tetrahedral
while the dominant peaks near 1288 cm-1 is assigned to the uas(P 0 2) stretching. The
intense bands at 1169 cm -1 and -664 cm -1 are due to the us(P 0 2) stretch and us(P0P)
stretch respectively. The S(P02) stretching appear at two peaks which are -352 and
-412 cm-1. There are also two shoulders with low intensity around -1129 cm-1 and -879
-1
2
cm which belong to the us(P 0 3) - and uas(P0P) groups. 0 n the other hand, there are
weak scattering peaks appearing at 1058 cm- 1 and 954 cm-1, are characterized as
pyrophosphate dimer (P 2 0 7)4- and orthophosphate monomer (P 0 4)3- species. The
5(P0P) linkage shows a low intensity at 409 cm -1 in the spectra region.
The band near 1320 cm
1
is caused by the symmetric stretching mode of the
terminal oxygen (P=0). The band’s intensity obviously becomes smaller with the
addition of Sr0, indicating decreased in double bond character and in the effective force
constant of (P=0) bond (Hussin et al., 2009).
The bands at 1288 and 1169 cm
1
are due to the asymmetric and symmetric
stretching modes of the two non-bridging oxygen atoms bonded to phosphorus atoms
(P 0 2) respectively (Brow, 2000). These two peaks including the peak near 664 cm -1 in
this spectra match with the value reported for metaphosphate N aP0 3 glass reported by
Nelson and Tallent (1985) and in zinc phosphate glass by Brow et al. (1995) which are
Intensity a.u
56
Wavelength, cm -1
Figure 4.9 (a): Raman spectra of xMg0-(50-x)Sr0-50P20 5 powder samples (with 0 < x
< 25 mol %).
Intensity a.u
57
Wavelength, cm-1
Figure 4.9 (b): Raman spectra of xMg0-(50-x)Sr0-50P20s powder samples (with 30 <
x < 50 mol %).
Table 4.3: Peak frequencies (cm"1) observed in the Raman spectra of the xMgO-(50-x)SrO-50P20s (with 0 < x < 50 mol %).
S(P02)
S(P-O-P)
us (P-O-P)
Uas(P-O-P)
t>s(P 0 4)3"
Us(P20 7)4-
us(P 0 3)2"
Us(P02)
Uas(P02)
0
352,410
450
664
879
954
1058
1129
1169
1288
132
5
352,411
450
664
878
953
1058
1130
1169
1288
132
10
352,411
450
689
879
954
1057
1130
1170
1289
132
15
352,412
451
689
879
954
1056
1131
1170
1289
132
20
353,412
451
689
879
955
1057
1131
1170
1289
132
25
352,413
451
689
879
954
1059
1132
1171
1289
132
30
353, 413
452
689
879
954
1059
1132
1171
1288
132
35
353,414
452
689
879
955
1058
1132
1170
1289
132
40
353,414
453
689
879
955
1058
1132
1171
1289
132
45
352,414
452
689
880
955
1059
1132
1170
1288
132
50
353,415
452
689
880
955
1059
1132
1170
1289
132
X
Us(P
mol %)
59
1267, 1165 and 689 cm-1. This proposes that the structure o f this composition
consists essentially o f a long phosphate chains based on P 0 3 - units. The uas(P 0 2)
band intensity increased as the amount o f M g0 is increased. This result is in
agreement with Hussin et al. (2009) which explaining the occurrence o f this
phenomenon due to the increase in the relative content o f the Q units.
The bands at 1058 and 1129 cm -1 correspond to the symmetric stretching
4_
1
2—
mode of (P 2 0 7) units in Q tetrahedral and symmetric terminal (P 0 3) symmetric
groups. These bands also appear in IR spectra where the pyrophosphate, (P 2 0 7)4—
3—
along with the orthophosphate, (P 0 4) units are dominant in the structures o f the
samples. The presence of bands at 954 cm —1 in this spectrum indicates isolated
3_
0
(P 0 4) group or Q phosphate units.
The influence of rare earth doping on 25M g0-25Sr0-50P20 5 is shown in
Figure 4.10 in the range 100 cm -1 to 4000 cm-1. From the spectra, two main peaks
3+
observed are us(P 0 2) and us(P-0-P). However, the inclusions of 1 mol % Eu and
3+
Dy ions give no notable changes in the spectra. It is suggested that small quantity
3+
3+
of Eu and Dy ions does not affect the Raman vibrations o f magnesium strontium
phosphate network and in agreement with Assaoudi and Ennaciri (1997).
4.3
Lum inescence Studies
The luminescence o f the rare earth ions in inorganic host has been
extensively investigated during the last few decades. As the phosphor material is
exposed to the ultraviolet light, the
excitation
energy is
converted into
electromagnetic radiation. This radiation can be observed in the visible range,
ultraviolet and infrared regions. Many applications deal with phosphor material such
Intensity (a.u)
60
Wavenumber, cm -1
Figure 4.10: Raman spectra o f 25M g0-25Sr0-50P20 5 (a) undoped (b) doped Eu3+
(c) doped Dy3+ (d) doped Eu3+ and Dy3+.
61
as vacuum fluorescent display, fluorescent lamps, plasma display and field emission
displays.
Among the rare earth ions, Eu
3+
3
and Dy
+
ions are mostly used in
luminescence due to the red and white characteristic (Xio and Yan, 2005). Therefore
3+
3+
in this section, the characteristics o f Eu and Dy ions in magnesium strontium
metaphosphate are presented.
4.3.1
Effect of E uropium Ion
Europium ions have been used widely as luminescence centre in phosphor
3+
materials and exhibit the red emission characteristic. In addition, Eu ions also have
a capability in emitting narrow band, almost monochromatic light and have long
after glow properties (Babu et al., 2007).
Figure 4.11 shows the luminescence spectra o f 25M g0-25Sr0-50P20 5 for
undoped and doped with 1 mol % Eu3+ samples. The 25M g0-25Sr0-50P20 5
3+
composition was chosen to be doped with Eu due to the optimum characteristic as
been discussed in previous section. The samples were excite using laser source of
325 nm and observed in the spectral range o f 300 - 800 nm.
For undoped sample, two broad bands peaking at 401 nm and 485 nm were
3+
observed. These broad bands belonged to the host emission. For Eu doped sample,
the intensity o f the host emission is reduced. The main emission peaks are due to the
3+
emission o f Eu ions as reported by Yuming et al. (2009). These peaks can be
5 7
assigned to the Do ^ F j(j = 0, 1, 2, 3, 4) transition with five peaks located at
568nm , 582nm , 605nm , 642 nm and 6 8 9 n m .T h em o st intensepeakat 605nm
Intensity (a.u)
62
Wavelength, nm
Figure 4.11: The luminescence spectra o f 25M g0-25Sr0-50P20 5 for (a) undoped
3+
and (b) doped with Eu .
63
corresponds to the hypersensitive transition between
5
7
D 0 and
F 2 level. This
transition; 7 F 2 level is due to electronic dipole which is induced by the lack of
inversion symmetry at the Eu3+ site and much stronger than magnetic dipole at 7 F 1
transitions.
In general, when Eu
3+
occupies a non symmetry sites and electric dipole is
allowed, the red emission become dominant (Yuming et al., 2009). However when
the magnetic dipole is allowed in the transition, the rare earth ion in the crystal
lattice occupies a site with inversion symmetry and produced an orange colour as
observed by Wang et al. (2009) in LaF 3 systems.
5 7
The luminescence spectra also shows that the D0^ F 1 j 2 j 3 j 4 transitions split
3+
into two or more lines as the Eu ions enter the crystal lattice. It is proposed that
3+
Eu ions substitute into Sr / Mg ions sites by formation o f Sr / Mg vacancies,
3+
therefore strontium magnesium phosphate host could accommodate Eu ions and
3+
stabilize Eu . The existence o f multiphase in X-ray diffraction analysis confirmed
that the Eu3+ ions tend to occupy the Sr2+ or Mg2+ sites in Sr(P03)2, SrMgP 2 0 7,
Mg 2 P 4 0
12
and Mg 2 P 2 0 7 phases and caused more emissions lines to occur. Zhou et
al. (2007) also reported that the trivalent europium ion occupies two points site in
BaGd 2 0 4 depending on the site symmetry.
The successive steps start with excitation from the ground singlet state to the
excited state by absorption from the laser source. The energised ions in the excite
state than pose a radiationless decay level by level reaching to the lowest energy
boundary o f the ground singlet state and thus forming a phosphorescence emission.
The energy level o fE u 3+ in 25M g0-25Sr0-50P20 5 is shown in Figure 4.12.
3+
In conclusion, luminescence deals with Eu ions acts as a red emission in
3+
magnesium strontium metaphosphate sample. Eu enters the crystals lattice and
64
5
7
became the strongest in the Do ^ F 2 transition at 605 nm. Thus, it is proposed to be
a good candidate for white LEDs applications as a red phosphor source.
Figure 4.12: The energy level o fE u 3+ in 25M g0-25Sr0-50P20 5.
65
4.3.2
Effect of D ysprosium Ion
The Dy
3+
is good activator because o f two dominated bands in the emission
spectra which are blue and yellow bands and its position depends strongly on the
crystal field o f the host lattice used (Blasse and Grabmaier, 1994). By adjusting the
3+
yellow to blue intensity ratio value, luminescence materials doped with Dy will
present white emission which can be used as a potential white phosphor.
Figure 4.13 shows the luminescence spectra o f undoped and Dy
3+
doped in
25Sr0-25M g0-50P20 5 as excited by a laser source o f 325 nm. For undoped sample,
two broad bands peaking at 401 nm and 485 nm were observed and belonged to the
3+
host emission. For Dy doped sample, two intense peaks appear at 477 nm and 564
nm. These peaks can be assigned to the
4
F 9/2 ^
6
H 15/2 and 4 F 9/2 ^
6
H 13/2 transitions
as reported by Yan and Huang (2007). The assignments o f the transitions are
indicated in the energy level diagram o f Dy3+ ion in Figure 4.14.
The electric dipole
H 13/2 transition with yellow emission is
prominent in the spectra. It indicates that Dy 3+ ion is located at the low symmetrical
4
F 9/2 ^
6
local site with no inversion centre. 0 n the other hand, the 4 F 9/2 ^
6
H 15/2 transition
with blue emission is magnetically allowed and hardly varies with the crystal field
strength around Dy 3+ ion. A weak red emission also appear at 665 nm corresponding
to the 4 F9 /2 ^
6
H 11/2 transition.
Two lines o f emission at 477 nm corresponding to the 4 F 9/2 ^
H 15/2 are
3+
observed in Figure 4.13. It is not suprising that the lines are splitting since Dy ion
6
supposely occupies the Sr or Mg site in magnesium strontium metaphosphate and
3+
Dy undergoes four different local environments with the Sr(P03)2, SrMgP 2 0 7,
Mg 2 P 4 0
12
and Mg 2 P 2 0 7 phases. The extrinsic energy levels from the defects is
important where Sr / Mg ion in the phosphate network induced all kind o f formation
of atomic defect such as Sr / Mg interstitial, vacancy or Frenkel pair (Shea, 1994).
Intensity (a.u)
66
Wavelength, nm
Figure 4.13: The luminescence spectra o f 25Sr0-25M g0-50P20 5 for (a) undoped
3+
and (b) doped with Dy .
67
9/2
20000
n7m
7
7
4
n4m
4
6
5
n5m
5
6
6
n1m
51
7
15000
6
F3/2 + 6 F 1/2
6
F5/2
(c
rg
e
n
w
7/2
H5/2
10000
f
ir
6
H7/2 + 6 F9/2
6
H9/2 + 6 F 11/2
6H 11/2
5000
1r
1r
6H 13/2
6H 15/2
Figure 4.14: The energy level ofD y 3+ in 25M g0-25Sr0-50P20 5.
From the Dy 3+ emission peaks present in Figure 4.14, it may be assumed that
3+
3+
the Dy ion caused the luminescence centre. Hussin et al. (2010) also observed Dy
in strontium magnesium pyrophosphate acts as a new self-active luminescence
3+
material instead o f acting as a trap centre. Dy act as a trap levels by capturing the
free holes. Then, it is release from the trap holes and recombines with the electron
and finally appears as a luminescence emission.
68
4.3.3
Effect of E uropium and D ysprosium Ions
The studies of the phosphor doped with europium and dysprosium ions have
been extensively explored in silicate and aluminate host phosphor. In silicate
2+
3+
network, Shi et al. (2007) reported that Eu and Dy produce a long lasting blue
emission while Kubo et al. (2005) proposed that calcium aluminate doped with
europium and dysprosium ions is suitable as a sensor material o f the fiber-optic
fluorescence thermometer application. Therefore in this section the luminescence
3+
3+
characteristic o f Eu
and Dy
in magnesium strontium metaphosphate are
presented.
Figure 4.15 shows the luminescence spectra o f Eu
3+
3+
and Dy ions doped in
25M g0-25Sr0-50P20 5. For purpose o f comparison, strontium metaphosphate and
3+
3+
magnesium metaphosphate doped with Eu and Dy respectively are included in
the Figure 4.15. The most obvious features are three groups o f sharp emission lines
at 472 nm, 565 nm and 609 nm. These peaks belonged to the 4 f 9/2 ^ 6 h 15/2 ( 4 7 2 Dm)
and 4 F9 /2 ^
6
H 13/2 (565 nm) for Dy3+ ions while red emission 5 D 0 ^
7
F 2 transition
appear at 609 nm. Zhang et al. (2005) also detected the characteristic emission from
Dy3+ which indicate that Dy3+ still existed in the phosphor when excited by
325 nm laser source.
In term o f intensities, the emission emits from magnesium strontium
metaphosphate is much higher compared to
strontium metaphosphate and
3+
3+
magnesium metaphosphate. This is probably due to Eu and Dy tend to occupy the
Sr2 + or Mg 2 + sites at different local sites which are magnesium strontium
metaphosphate, strontium metaphosphate and magnesium metaphosphate. Aizawa et
al. (2006) also reported that the luminescence peaks’ shape and position are
depended on the crystal structure o f the host materials. This is similar explanation as
discussed for Eu 3+ and Dy3+ in section 4.3.1 and 4.3.2 earlier. Magnesium strontium
3+
3+
metaphosphate doped with Eu and Dy is proposed to have a better luminescence
69
characteristic
as
compared
to
strontium
metaphosphate
and
magnesium
metaphosphate.
Wavelength, nm
Figure 4.15: The luminescence spectra o f Eu
(b)
50
M g 0 -5 0 P 2 0 7 (c)
2 5
3+
Mg 0 -2 5 Sr0 - 5 0 P 2 0 5
and Dy
3+
doped in (a) 50Sr0-50P20 7
70
The energy level o f Eu3+ and Dy3+ doped in metaphosphate is shown in
3+
Figure 4.16. During excitation, the energy is absorbed by direct transition from Eu
and release back to the 4 f ground state. As proposed by Zhang et al. (2005), Dy3+ ion
cannot be excited but as the excitation energy is absorbed by the band gap transition,
3+
3+
the excitation can be relaxed to Eu
and Dy . This process leads to the
3+
3+
luminescence emission by Eu and Dy in magnesium strontium metaphosphate
system.
5
D2
Di
5
D 0 Energy
Transfer
n2m
2
7
4
n5m
5
6
5
6 H 7 /2 + 6 F 9/2
5
6 H 9/2 + 6 F h /2
4
6 H h /2
3
1f
2
1
1
0
Figure 4.16: The energy level model o f Eu
metaphosphate.
3+
and Dy
3+
in magnesium strontium
CHAPTER 5
CONCLUSION AND RECOMM ENDATIONS
5.1
Summ ary
Metaphosphate phosphors which are physically stable have been successfully
prepared
via
solid
state
reaction
method.
Magnesium
and
strontium
were
homogeneously mixed followed by a two step sintering process at 300oC and 900oC.
The findings reported in this study are solely based on the structural and luminescence
properties of magnesium strontium metaphosphate system.
From the X-ray diffraction pattern, all the powders were confirmed to be in
polycrystalline phase and four main phase transformation appeared which are Sr(P03)2,
SrMgP2 0 7, Mg2 P 4 0 i 2 and Mg 2 P2 0 7. By analyzing samples at different sintering
temperature, it was found that the optimum sintering temperature is 900oC. The
3+
optimum sample composition chosen is 25M g0-25Sr0-50P20 5 to be doped with Eu
and Dy3+. However the addition of Eu3+ and Dy3+ resulted in no changes observed in the
phase formation.
72
The results of Infrared and Raman studies showed that magnesium strontium
metaphosphate system consist of a main network of Q 2 tetrahedral units. There are also
low vibrations belong to the Q 1 and Q0 tetrahedral units. There were four main region
observed in both of the spectra which are P = 0 vibration (1320 cm-1), non-bridging P 0 2
groups (1200 - 1170 cm-1), P 0 3 and P 0 4 groups (1160 - 950 cm-1) and the region
known as P-0-P terminals (950 - 704 cm-1). The infrared spectra at higher frequency
showed two broad peaks at around 3422 cm -1 and 1644 cm-1. These peaks correspond to
the stretching vibration of water molecules (vH2 0 ) and deformation of 0-H groups
(JH 2 0). It is suggested that the introduction of Sr0 and M g0 as modifiers reduced the
hygroscopic properties of the phosphate since the band’s intensity is reduced by the
inclusion of these modifiers.
The influence of rare earth doping on metaphosphate structure proposed that the
3+
3+
inclusions of 1 mol% Eu and Dy ions gave no notable changes in the IR and Raman
spectra. It is suggested that the small quantity of Eu3+ and Dy3+ ions does not affect the
local structure of magnesium strontium metaphosphate network.
Luminescence deals with Eu3+ ions acts as a red emission in magnesium
3+
strontium metaphosphate sample. Eu enters the crystals lattice and became the
strongest in the 5 D 0 ^
7
F2 transition at 605 nm. Thus it is proposed to be a good
candidate for white LEDs applications as a red phosphor source.
In addition, luminescence properties by Dy
3+
doped sample showed two intense
peaks appearing at 477 nm and 564 nm. These peaks are assigned to the
with yellow emission and 4 F9/2 ^
4
F9/2 ^
6
H 15/2
H 13/2 blue emission. From the Dy3+ emission peaks
3+
presented, it may be assumed that the Dy ion caused the luminescence centre and in
6
agreement with Hussin et al. (2010).
73
Magnesium strontium metaphosphate doped with Eu
3+
3+
and Dy is proposed to
have a better luminescence characteristic as compared to strontium metaphosphate and
magnesium metaphosphate. In term of intensities, the emission emits from strontium
magnesium metaphosphate is much higher compared to strontium metaphosphate and
magnesium metaphosphate. This probably happened when Eu3+ and Dy3+ tend to
occupy the Sr2 + or Mg 2 + sites at different crystals site of magnesium strontium
metaphosphate, strontium metaphosphate and magnesium metaphosphate.
In conclusion, magnesium strontium metaphosphate phosphors have showed a
remarkable structural and luminescence properties which is very useful in the plasma
display applications.
5.2
Future Study
It is recommended that further characterization by nuclear magnetic resonance
(NMR), transmission electron microscopy (TEM) and scanning electron microcopy
(SEM) should be attempted. Those measurements will give more information on the
structural studies of metaphosphate phosphor. Moreover, the thermolumnescence and
decay curve of the afterglow phosphors should be studied in future so that the effect of
defect and the decay lifetime could be obtained.
It is also suggested that phosphate at different concentration composition doped
with various rare earth should be studied in the future.
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APPENDIX A
THE SAMPLES CALCULATION
The samples calculation based on xM g0-(50-x)Sr0-50P20s (with 0 < x < 5 0
mol %) systems are shown below.
Based On
S rC 03
MgO
2(H3P04)
TOTAL MASS
Actual Mass for 10g
2(H3PO4)
Based On
SrC03
MgO
2(H3PO4)
TOTAL MASS
Actual Mass for 10g
2(H3PO4)
%mol
147.6292
40.3044
195.9902
7.38146
18.13698
97.9951
123.5135
1g
0.059762
0.146842
0.793396
1
10g
5%
45%
50%
14.76292
16.12176
97.9951
128.8798
1g
0.114548
0.125091
0.760361
1
10g
10%
40%
50%
0.597624
1.46842
7.933956
10
9.124049
%mol
147.6292
40.3044
195.9902
8.744146
1.14548
1.250915
7.603605
10
83
Based On
SrCO3
MgO
2(H3PO4)
TOT>L M>SS
>ctual Mass for 10g
2(H3PO4)
Based On
SrCO3
MgO
2(H3PO4)
TOT>L M>SS
>ctual Mass for 10g
2(H3PO4)
Based On
SrCO3
MgO
2(H3PO4)
TOT>L M>SS
>ctual Mass for 10g
2(H3PO4)
Based On
SrCO3
MgO
2(H3PO4)
TOT>L M>SS
>ctual Mass for 10g
2(H3PO4)
%mol
147.6292
40.3044
195.9902
>ctual Mass for 10g
2(H3PO4)
29.52584
12.09132
97.9951
139.6123
1g
0.211485
0.086606
0.701909
1
10g
20%
30%
50%
10g
25%
25%
50%
36.9073
10.0761
97.9951
144.9785
1g
0.254571
0.069501
0.675928
1
44.28876
8.06088
97.9951
150.3447
1g
0.294581
0.053616
0.651803
1
10g
30%
20%
50%
51.67022
6.04566
97.9951
155.711
1g
0.331834
0.038826
0.62934
1
10g
35%
15%
50%
1.649537
1.050798
7.299665
10
8.394615
%mol
147.6292
40.3044
195.9902
2.114846
0.866064
7.01909
10
8.071953
%mol
147.6292
40.3044
195.9902
2.545709
0.695007
6.759285
10
7.773178
%mol
147.6292
40.3044
195.9902
2.945814
0.53616
6.518027
10
7.49573
Based On
SrCO3
MgO
2(H3PO4)
TOT>L M>SS
22.14438
14.10654
97.9951
134.246
1g
0.164954
0.10508
0.729967
1
10g
15%
35%
50%
%mol
147.6292
40.3044
195.9902
7.237406
3.318341
0.388262
6.293397
10
84
T oo M s
O h Q C
>
L
- “p
O ° O
M ^
>
C/3
C/3
Based On
>ctual Mass for 10g
2(H3PO4)
%mol
147.6292
40.3044
195.9902
40%
10%
50%
45%
5%
50%
S
S
>
"
9 7$
9 O ” 1­
v eg ^ O
C
O S C
M 1—
>ctual Mass for 10g
2(H3PO4)
Based On
SrCO3
MgO
2(H3PO4)
TOT>L M>SS
>ctual Mass for 10g
2(H3PO4)
1g
10g
%mol
147.6292
40.3044
195.9902
Based On
SrCO3
MgO
2(H3PO4)
TOT>L M>SS
10g
66.43314
2.01522
97.9951
166.4435
1g
0.399133
0.012108
0.588759
1
3.666048
0.250218
6.083734
10
6.996294
Based On
>ctual Mass for 10g
2(H3PO4)
1g
0.366605
0.025022
0.608373
1
10g
59.05168
4.03044
97.9951
161.0772
3.991334
0.121075
5.887591
10
6.77073
%mol
147.6292
40.3044
195.9902
50%
0%
50%
73.8146
0
97.9951
171.8097
0%
50%
50%
0
20.1522
97.9951
118.1473
0.42963
0
0.57037
1
4.2963
0
5.7037
10
6.559255
%mol
147.6292
40.3044
195.9902
9.538463
1g
0
0.170568
0.829432
1
10g
0
1.705684
8.294316
10
APPENDIX B
PUBLICATIONS
Hamdan, S., Hussin, R., Salim, M.A., Husin, M.S, Abdul Halim, D.N.F. and Abdullah,
M.S. (2011). The morphology and composition of strontium calcium aluminate matrix
3+
doped with Dy . Materials Science and Technology. Volume 27, Num 1, 232-234 (3).
Hussin, R., Hamdan, S., Halim, D.N. F.A. and Husin, M.S. (2010). The Origin of
Emission in Strontium Magnesium Pyrophosphate doped with Dy 2 O3. Material
ChemistryandPhysics. 121 37-41.
Hussin, R., Husin, M.S., Halim, D.N. F.A. and Hamdan, S. (2010). Vibrational Study of
Crystalline Phase Strontium Magnesium Phosphates doped with Eu 2 O3. Solid State
Science and Technology. Volume 18, No 1, 288-301.
Hussin, R., Halim, D.N. F.A., Husin, M.S., Hamdan, S. and Yusof, M.N.M.Y. (2009).
Luminescence Properties of 30Sr0-30M g0-40P20s Doped With Dy2 O3. Solid State
Science and Technology. Volume 17, No 2, 123-132.