Journal of Non-Crystalline Solids 326&327 (2003) 125–129
www.elsevier.com/locate/jnoncrysol
Structural investigations of the Se–Ag–I system
T. Petkova
a,*
, M. Mitkova
a,b
, Mir. Vlcek c, S. Vassilev
a
a
c
Central Laboratory for Electrochemical Power Sources, Bulgarian Academy of Sciences,
Acad. G. Bonchev Str. Bl 11, 1113 Sofia, Bulgaria
b
Center for Solid State Electronics Research, Arizona State University, Tempe, AZ 85287-6206, USA
Department of General and Inorganic Chemistry, University of Pardubice, Nam. Cs. Legii 565, Pardubice 532 10, Czech Republic
Abstract
Glasses from the Se–Ag–I system have been investigated. Structural information is gathered based on results col delected by a combination of several types of diffraction measurements. First coordination sphere at r ¼ 2:3–2:5 A
termined by radial distribution function calculations can be assumed to be composed by Se–Ag and Ag–I correlations
in the network units and Se–Se correlation in the Se cluster units. The interatomic distances and average bond angles
decrease with the introduction of additives to selenium. The Raman spectra reveal that the introduction of silver and
iodine, in particular, brings about a shift of the Se chain stretching mode towards lower wave numbers (251–236 cm1 ).
Apparently, the newly formed structure after the introduction of additives into selenium is pretty compact and is
becoming more covalent with a stronger interchain interaction. Depending on the concentration of additives the
breathing modes of Ag–Se and Ag–I bonds also can be seen.
Ó 2003 Elsevier B.V. All rights reserved.
PACS: 61.43.D; 61.80.C; 73.61.A; 78.30.L
1. Introduction
Selenium, which is known as one of the major
glass formers among the chalcogenide glasses, has
a tendency to form atomic chains built up by Se8 fragments and chain segments [1]. Introduction of
metallic atoms brings about polymerization of the
Se matrix and formation of three-dimensional
structural units due to the usually higher coordination of the metals. Special attention has been
*
Corresponding author. Tel.: +359-2 724 339; fax: +359-2
722 544.
E-mail address: t_petkova@hotmail.com (T. Petkova).
paid to systems containing silver [2,3]. On the
other hand, introduction of halogen elements to
the Se chains causes an increase of glassy matrix
floppiness as halogen atoms can bond in the chains
and are also responsible for chains fragmentation
by acting as chain terminators because they are
one-fold coordinated [4]. The Se–Ag–I system,
which contains both metal and halogen elements,
offers a challenging opportunity to explore changes in the structure appearing in the pure chalcogen matrix of such a complex system. Further
more, recently Boolchand and Bresser [5] has
shown that both products of eventual reactions in
these systems – Ag2 Se and AgI – can vitrify since
the mechanically effective connectivity for them is
0022-3093/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved.
doi:10.1016/S0022-3093(03)00390-9
126
T. Petkova et al. / Journal of Non-Crystalline Solids 326&327 (2003) 125–129
2.26 and 2, respectively, which suggests that the
morphological structure of the AgI glass may be
similar to that of the Se glass.
Our previous investigations on the Se–Ag–I
system indicate formation of glasses that include
up to 23 at.% silver and 13 at.% iodine [6]. The
glasses have only one glass-transition temperature
that suggests formation of a homogeneous or
nano-phase separated structure, by which the
small sizes of the phase-separated species does not
affect the properties of these glasses on a macroscopic scale. The glass-transition temperature is
pretty much low and it decreases with the addition
of overstoichiometric iodine in respect to the AgI
composition. This leads us to believe that the halide becomes part of the backbone of the glass and
by being one-fold coordinated it lowers its main
coordination.
In this work we give a more detailed review of
the structural organization of the glasses from the
Se–Ag–I system emphasizing the changes that
occur in the Se matrix. We are using results given
by X-ray diffraction measurements. To complement these results we studied also the Raman
spectra of these glasses, which relate directly to
the interaction between atoms within structural
units.
2. Experimental procedure and calculations
Glasses
with
composition
Se1x (AgI)x ,
Se1x Ag5 Ix and Se1x Ag10 Ix (x ¼ 5, 10, 15 at.%)
were synthesized as described in our previous papers [6]. X-ray diffraction patterns of the powder
samples (particle size less then 5 m) were obtained
with an X-ray diffractometer Philips h–2h Bragg–
Brentano geometry using CuKa radiation (k ¼
) and mounted graphite monochromator
1:54178 A
for diffracted beam. The diffraction data were
collected for 60 s at each 0.2° step width over a 2
range from 5° to 150° in the range of scattering
1 ðQ ¼
vector magnitudes Q between 0.4 and 8 A
4p sin h=kÞ. All the X-ray investigations were performed at ambient temperature. The diffraction
intensities were corrected for the background, incoherent (polarization and absorption) and multiple scattering, respectively, in the usual way in
order to eliminate the part of radiation which does
not carry structural information. The spectra were
scanned at a constant rate. After a Fourier transformation the reduced radial function GðrÞ was
obtained:
GðrÞ ¼ 4prfqðrÞ q0 g;
ð1Þ
where q0 is average atom density of the alloy and
qðrÞ is the atom density as a function of r. The
radial distribution function can be written as
RDF ¼ 4pr2 qðrÞ ¼ rGðrÞ þ 4pr2 q0 :
ð2Þ
A parameter of great interest is the area enclosed
under the first radial distribution function (RDF)
peak as it represents the average coordination
number (N ). The average coordination number in
a spherical shell between radius r1 and r2 around
any given atom can be calculated as
Z r0
N¼
4pr2 qðrÞ dr;
ð3Þ
r0
where r0 is a lower limit of r below which qðrÞ is
zero and r0 is the first minimum of 4pr2 qðrÞ.
The position of the first peak gives a value for
the nearest-neighbour bond length, r1 , and similarly the position of the second peak gives the nextneighbour distance, r2 ; a knowledge of both
immediately gives a value for the bond angle H:
H ¼ 2 arcsinðr2 =2r1 Þ:
ð4Þ
The RDF yields only a limited amount of information, restricted essentially to the local structure around a given atom, i.e. bond lengths and
bond angles. The structural origin of more distant
correlations corresponding to higher order peaks
in the RDF cannot be obtained directly, but only
in conjunction with additional information, which
may be gathered, for example, from spectroscopic
data.
The Raman spectroscopy investigations were
performed with Fourier transformation IR spectrophotometer IFS55 with a Raman accessory
FRA 106 Bruker, Germany. The laser irradiation
at the wavelength of 1.06 lm with an output power
of 90 mW was used for the excitation of the Raman spectra. This wavelength value was essential
because irradiation of samples in this range causes
no detectable photostructural transformations
T. Petkova et al. / Journal of Non-Crystalline Solids 326&327(2003) 125–129
127
within the scale of 100 scans. The resolution of the
Raman spectrometer was 1 cm1 .
Se75Ag15 I10
The positions of the first and second peaks in
the radial distribution function imply the interatomic distance in Se–Ag–I glasses (Fig. 1). It is
shown that when silver and iodine are presented by
equal quantities in the glass, the position of the
first peak in RDF shifts gradually with increasing
, due to the forAgI content from 2.34 to 2.46 A
mation of heteropolar bonds (Ag–Se and Ag–I).
When the iodine content exceeds that of Ag or
contrawise, the silver concentration is bigger in
respect to that of iodine, we do not observe any
changes in first RDF peak. A small change in the
next peak position can be indicated occurring
probably due to the direct Se–I bonding. However,
evidences for these bonds could not be obtained by
X-ray diffraction investigation alone.
The Raman spectra of the investigated glasses
are shown on Fig. 2. They are compared with
the spectrum of the amorphous selenium, which
4 πr 2ρ(r )
Se75Ag10I15
Se75Ag15I10
Se70 Ag15 I15
Se80Ag10I 10
Se90Ag5 I5
Se
0
2
4
6
8
10
r(A)
Fig. 1. Radial distribution curves for different glassy compositions.
Intensity (arb.units)
3. Results
Se75Ag10I15
Se70 Ag15I15
Se80 Ag10 I10
Se90Ag5I5
Se
50 100 150 200 250 300 350
Raman shift,cm-1
Fig. 2. Raman spectra of the investigated glasses, the Raman
spectrum of pure Se is given for comparison.
features well expressed fundamental vibrational
modes at 250 and 112 cm1 . These two modes
characterize the presence of Se8 fragments that
have cis-coupling and give rise to the features of
bond stretching vibration at 250 cm1 in the plane
and the bond bending motion with a frequency of
112 cm1 perpendicular to the plane and the chain
segments with trans-coupling that has only one A1
mode, a bond-stretching vibration with an inplane motion, occurring at 250 cm1 as well [7].
The mode at 250 cm1 has a shoulder at lower
wave numbers that has been deconvoluted at the
fitting procedure with a peak at 235 cm1 . It is
characteristic for the trigonal form of Se [8]. When
5–15 at.% silver and iodine are progressively introduced to the Se matrix, the mode at 235 cm1
increases in amplitude at the expense of the intensity of the mode at 250 cm1 and becomes the
only characteristic feature of Se when higher
amounts of silver and iodine are introduced into
the glass as the mode at 112 cm1 does not appear
in those cases. At this point, however, new modes
emerge like those at 144 cm1 manifesting a formation of Ag2 Se, the modes of AgI (84 and
128
T. Petkova et al. / Journal of Non-Crystalline Solids 326&327 (2003) 125–129
123 cm1 ) [9] and iodine (182 and 191 cm1 ) depending on the composition of glasses.
4. Discussion
The position and intensity of the third and the
next maximum on the RDF curves can be assumed
as related mainly to the structure of Se atoms. We
gives rise to
can speculate that the peak at 4.5 A
chain-like features, whereas the absence of this
peak implies ring-like-fragment morphology for
pure Se. The peak diminishes suggesting that the
chain-like features are partially destroyed.
The calculated values for bond angles show no
significant differences. The average value is 95–
100°. Only pure Se shows bigger H (105°). A decrease in the bond angle values is probably due to
the formation of new structural units and a more
compact structure of the investigated glasses with
increasing additives to Se (Table 1).
We consider the character of the RDF as a
probability function and the position of the peak
can be interpreted as the average distance of the
Table 1
Structure parameters of Se–Ag–I glasses
)
)
Composition
r1 (A
r2 (A
Se
Se90 Ag5 I5
Se80 Ag10 I10
Se75 Ag10 I15
Se75 Ag15 I10
Se70 Ag15 I15
2.34
2.36
2.4
2.4
2.4
2.46
3.72
3.58
3.54
3.54
3.56
3.78
H
105.9
98.58
95.12
95.12
95.46
100.35
different coordination spheres to an arbitrary atom
taken as a reference origin. The most probable
three kinds of bonds Se–Se, Se–Ag and Ag–I
contribute mainly to the diffraction spectrum and
the first diffraction peak. The position of the first
represents the average
maximum at 2.34–2.46 A
distance between first neighbours. We fit the experimental RDF using four Gaussian functions
(Ag–Se bond), 2.33 and 3.69
with maxima 2.67 A
(Se–Se) and 2.55 A
(Ag–I). The results from the
A
fitting procedure are given in Table 2. The values
of Ag–Se bond lengths are much smaller than the
þ
) and
sum of the ionic radii of Ag (rAg
1:26 A
2
), which indicates a substantial
Se(rSe 1:91 A
Table 2
Results of T ðrÞ analysis
Composition
Correlations
Se
Se–Se
Se–Se (second
Se–Se
Ag–I
Ag–Se
Se–Se (second
Se–Se
Ag–I
Ag–Se
Se–Se (second
Se–Se
Ag–I
Ag–Se
Se–Se (second
Se–Se
Ag–I
Ag–Se
Se–Se (second
Se–Se
Ag–I
Ag–Se
Se–Se (second
Se90 Ag5 I5
Se80 Ag10 I10
Se75 Ag10 I15
Se75 Ag15 I10
Se70 Ag15 I15
neighbour)
neighbour)
neighbour)
neighbour)
neighbour)
neighbour)
)
Distance (A
Partial coord. number
2.33
3.68
2.3
2.58
2.79
3.62
2.3
2.55
2.77
3.66
2.33
2.55
2.67
3.68
2.33
2.55
2.67
3.68
3.33
2.56
2.67
3.67
4.6
11.2
1.65
0.96
0.03
4.04
1.74
0.93
0.2
5.25
2.65
0.55
0.23
6.72
1.99
0.47
0.49
4.89
0.7
0.59
0.33
3.78
T. Petkova et al. / Journal of Non-Crystalline Solids 326&327(2003) 125–129
covalent character of these bonds. This is also
confirmed by the Raman results since the formation of a structure close to trigonal chains defined
by the development of the mode at 235 cm1 is
correlated to a formation of more covalent structure with stronger interchain forces. The appearance of the peak at 235 cm1 is always
accompanied with an absence of the peak at 112
cm1 . As pointed out by Lucovsky [8], this feature
is related to the presence of structural units with
cis-coupling configuration having breathing modes
at 112 and 250 cm1 . These are the frequencies
with highest density of states characteristic for
selenium with a network type of structure [10].
The molecular dynamic simulations confirm the
Raman activity of these units also with high
proximity [11]. In principle, these cis-coupling
configurations can form a closed structure since
due to the alternation of the bond angles in sign [8]
these fragments can form a ring. However the fact
that they disappear after introducing additives to
selenium reveals the formation of more ÔopenÕ
structure built up by trans-coupling configurations
that maintain the sense of the dihedral angles
leading to helical arrangement of a potentially infinite extent [8]. The development of this structure
is related closer to the introduction of iodine in the
glasses while even at a higher silver concentration
(15 at.%) but less iodine (10 at.%) the initial a-Se
structure manifested by the mode at 250 cm1 is
still present in the Raman spectrum (Fig. 2). The
earlier results of one of us show that when introduced in the Ge–Se matrix, silver reacts with Se
from the Se chains to form Ag2 Se but this does not
affect the structure of the free selenium units [12].
Indeed, the chemical bonding between the three
elements of the glass is a consequence of two effects: (1) A bond strength energy [13] that requires
Ag to predominantly bond to selenium instead of
iodine as this process requires lower energy (202.5
kJ/mol vs. 234 kJ/mol). This restricts formation of
Ag–I bonds and leaves some free iodine atoms. (2)
On the other hand, these free iodine atoms clamp
the backbone locally [14] because of the larger
covalent radii of the iodine additive. As a result, a
much more compact structure occurs that is
manifested by the occurrence and growth of the Se
chain mode at 235 cm1 . Such an effect has been
129
described also for the Ge–S–I system [15], where
iodine induces rigidity transition in the system and
affects the self-organization of the random network.
5. Conclusions
Our studies on the structure of Se–Ag–I glasses
indicate that introduction of silver and iodine to
the Se-matrix leads to formation of more open and
closely packed structure.
Acknowledgements
The authors acknowledge the helpful discussions with Professor P. Boolchand during the
preparation of the manuscript.
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