Hội nghị Vật lý chất rắn và Khoa học vật liệu toàn quốc lần thứ 6 (SPMS-2009)-Đà nẵng 8-10/11/2009
PHỤ LỤC
THE MAGNETIC ANOMALY OF
THE DOUBLE RARE-EARTH ELEMENTS PEROVSKITE
COMPOUNDS
(La1-xYx )FeO3 AND (La1-yNdy)FeO3
Dang Le Minh1, Nguyen Minh Tuan1 Nguyen Thi Thuy2
Nguyen Phu Thuy3, Nguyen Thanh Trung4
1
Hanoi University of Science,VNU, Hanoi
2
Hue Pedagogical University-Hue University
3
College of Technology, VNU, Hanoi and ITIMS, HUT, Hanoi
4
Delft University – Netherland
Abstract. The double rare-earth elements perovskite compounds of (La1xYx)FeO3 and (La1-yNdy)FeO3 (x, y = 0.00; 0.15; 0.35; 0.55; 1.00 ) were
prepared by ceramic method. Magnetic measurements have shown that
the samples are ferromagnetic. The magnetic hysteresis loops of two
series of samples measured from 5K to 300K have the anomalous shapes.
It can be suggested that the magnetic anisotropy and temperature
influence on the magnetization process are the reasons of the observed
phenomena in these samples.
Keywords : Magnetic anomaly, Double rare-earth elements,
Magnetization, Ferromagnetic, Hysteresis loop.
INTRODUCTION
Perovskite-type oxides with the general formula ABO3 , where A can be
an alkali, alkaline earth, or a lanthanide metal and B can be a transition metal,
play an important role in the preparation of catalysts for specific applications.
Their composition can be varied over a wide range by partial substitution of
cation in position A and B yielding compound of the formula
(AxA’1-x)(ByB’1-y)O3. Particularly, lanthanium transition metal perovskite
oxides, which are denoted as the chemical formula LaMBO3
(MB = 3d transition metal), have been attracting the scientific interests due to
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substitution effects of various impurity ions into the perovskite A and B sites.
The LaFeO3-based oxides is characterized as an orthorhombic perovskite
structure. They are important for cathode and interconnection materials for
solid oxide fuel cell (SOFCs), gas sensors, and humidity sensors [1,2,3,4].
In our previous report [5], the study of the properties of LaFeO 3 has
shown that it is a semiconductor with weak ferromagnetic behavior. In this
work, the double rare-earth elements perovskite compounds (La1-xYx)FeO3
and (La1-yNdy)FeO3 (x, y = 0, 0.15, 0.35, 0.55, 1.00) were prepared and the
influence of second rare-earth elements in the A-site on the magnetic
properties of the samples has been investigated.
EXPERIMENTAL
(La1-xYx)FeO3 and (La1-yNdy)FeO3 (x, y = 0, 0.15, 0.35, 0.55, 1.00 ) were
prepared using the conventional solid-state synthesis technique. La2O3, Nd2O3
,Y2O3 and Fe2O3 powders were used as starting materials. The mixed powder
was presintered in air at 10000C for 10h. The presintered powder was pressed
into pellets which were finally sintered at 1230 0C for 5h. Powder X-ray
diffraction (XRD) with Cu-Kα radiation was used to measure these pellets
and investigate their crystal structures. Magnetic measurements were carried
out using a superconducting quantum interference device (SQUID)
magnetometer (Quantum Design) under the applied magnetic field of 5T in
the temperature range between 5K and 300K.
RESULTS AND DISCUSSION
The X-ray diffraction analysis of (La1-xYx)FeO3 and (La1-yNdy)FeO3
indicates that all samples are single phase of the orthorhombic structure. The
lattice parameters are listed in Table 1. The requirement for stability of this
orthorferrite is that the Goldschmidt tolerance factor (t) should be of nearly
unity, and (t) is defined via RA+ R0 = t 2( RB  RO )
Where RA, RB and R0 are radii of ions at A , B sites and oxygen ion,
respectively.
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Table 1. The lattice parameters (a, b, c, V), tolerance factor (t) , and RA, RB
radii in (La1-xYx)FeO3
x
Compound
0
LaFeO3
0.15 La0.85Y0.15FeO3
0.35 La0.65Y0.35FeO3
0.55 La0.45Y0.55FeO3
1
YFeO3
a, A0
b, A0
c, A0
V,(A0)3
t
5.56
5.54
5.55
5.55
5.28
5.55
5.57
5.52
5.45
5.59
7.86
7.84
7.84
7.83
7.60
242.24
241.93
240.19
236.84
224.32
0.921
0.913
0.903
0.892
0.870
RA
A0
1.36
1.33
1.3
1.27
1.19
RB
A0
0.645
0.645
0.645
0.645
0.645
Table 2. The lattice parameters (a, b, c, V), tolerance factor (t) , and RA, RB
radii in (La1-yNdy)FeO3
y
Compound
0
LaFeO3
0.15 La0.85Nd0.15FeO3
0.35 La0.65Nd0.35FeO3
0.55 La0.45Nd0.55FeO3
1
NdFeO3
a, A0 b, A0 c, A0
V,(A0)3
t
5.56
5.55
5.54
5.53
5.58
242.24
241.67
241.06
239.70
235.99
0.921
0.917
0.911
0.906
0.894
5.55
5.54
5.55
5.55
5.45
7.85
7.86
7.84
7.81
7.76
RA
A0
1.36
1.35
1.33
1.31
1.27
RB
A0
0.645
0.645
0.645
0.645
0.645
It can be seen from Tab.1 that the volume of the unit cell decreases with
increasing the Y and Ni content because the radius of Nd3+(0.109Ǻ) and Y3+
(0.097 Ǻ) are both smaller than that of La3+(0.114Ǻ).
However the structure distorsion caused by Nd substitution is much less than
that caused by Y substitution. This fact should strongly influence on the
magnetic property of the samples.
The M(H) curves of two series of (La1-xYx)FeO3 and (La1-yNdy)FeO3
obtained from SQUID magnetometer have shown in Figure 1, 2 and 3. It can
be noted that both of them are anomalous in different ways.
Concerning the hysteresis loops of the (La1-yNdy)FeO3 samples (Fig.1),
it can be seen that while the magnetization curve measured at 5K raises
strongly with increasing of y from y = 0.15 to y = 0.55 (Fig.1b,c,d) making
the magnetization process much easier with larger Nd content, the curves
measured in the (100K-300K) range (Fig.1b,c,d) and (5K-300K) range with
y = 0 (Fig.1a) are nearly not changed for all compositions of the samples.
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We explain this anomaly by supporting that the total anisotropy energy of
this series composes of two compositions coming from substituted Nd and
from Fe-sub-lattice and the Nd-anisotropy fast decreases with increasing of
temperature while the Fe-anisotropy is nearly constant in the whole range of
(5K-300K). That makes the total anisotropy and consequently the
magnetization processes drastically change at 5K from sample to sample
while it is nearly the same in the (100K-300K) range.
As far as the (La1-xYx)FeO3 series is concerned (Figs.2 and 3) it can note
firstly that they become very ”soft” in the small magnetic field for all samples
which x≠0 while at higher field they are all hardly saturated. In comparison
with the case of (La1-yNdy)FeO3 (Fig.1) , the hysteresis loops of the samples
(La1-xYx)FeO3 are more rectangular (Fig.2,3). All of them have the
magnetization transition at a certain magnetic field called the critical field
(Hcrit). At 5K the virgin magnetization curve is nearly coincided with the
decreasing branch of the hysteresis loop (Fig.1a).
These anomalous behaviors can be understood by suggesting that the Y
substitution for La causes the distortion of the crystal field created by the
ligands of the Fe ions making the low order anisotropy constant of this sublattice drastically reduces and the Fe-lattice anisotropy is mainly due to the
higher order anisotropy constant. It makes the materials become “soft” and
the magnetization transition at Hcrit was occurred. This transition can be due to
the abrupt change of the easy axis which is determined by this anisotropy
constant under the influence of the magnetic field.
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5K
5K
100K-300K
100K-300K
(La1-yNdy)FeO3
y=0.00
(a)
(La1-yNdy)FeO3
y=0.15
(b)
5K
5K
100K-300K
(La1-yNdy)FeO3
y=0.35
(c)
100K-300K
(La1-yNdy)FeO3
y=0.55
(d)
Fig.1 The curves M(H) of the samples of (La1-yNdy)FeO3
y=0 (a); y=0.15 (b); y=0.35(c); y=0.55(d)
(a)
Fig. 2 The curves M(H) of the samples of (La1-xYx)FeO3
y=0.15 (a); y=0.35(b)
(b)
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(b)
(a)
(c)
(d)
Fig. 3 The curves M(H) of the samples of (La1-xYx)FeO3 , x =1
measured at 5K(a); 100K(b); 200K(c); 300K(d)
CONCLUSION
The perovskite lattice of orthorhombic LaFeO3 was distorted by Nd and
Y substitution and the tolerance factor varies far from unity causing the
decrease of unit cell volume and the magnetic anomaly.
Interesting magnetic anomalies observed in both sample series have been
qualitatively explained based on the differences between the anisotropy of the
substituted samples and that of the hot sample LaFeO3. Of course, in order to
confirm these explanation, the measurements on single crystals are strongly
recommended.
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Acknowledgments
The authors would like to thank “National Foundation for Science and
Technology (NAFOSTED)”, with the project code “103.03.69.09” for
financial support.
References
1. G. Shbbir, A.H Qureshi, K. Saeed Materials Letters 60 (2006) 37063709
2. Ivar Warnhus et all Solid State Ionics 178 (2007) 907-914
3. Define Bayraktar et all Journal of the European Ceramic Society 27
(2007) 2445-2461
4. Peter Dinka, Alexander S.Mukasyan
Journal of Power Sources 67 (2007) 472-481
5. Dang Le Minh, Nguyen Van Du and Nguyen Thi Thuy
Proceeding of the Eleventh Vietnamese-German Seminar on Physics and
Engineering
Nha Trang City 31 March – 05 April, 2008, p.267-270.
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THE ELECTRIC PROPERTY OF THE DOUBLE RARE-EARTH
ELEMENTS PEROVSKITE COMPOUND (La1-xYx )FeO3
Nguyen Thi Thuy 2 , Nguyen Minh Tuan 1 , Dang Le Minh 1, Nguyen Phu
Thuy 3
1
Hanoi University of Science-VNU
2
Hue Pedagogical University-Hue University
3
College of Technology-VNU of Hanoi and ITIMS-HUT, Hanoi
Abstract : The double rare-earth elements perovskite compound of
(La1-xYx)FeO3 (x = 0; 0,15, 0,25; 0,35; 0,45; 0,55; 1 ) were prepared by
ceramic method. Measurement of the electric property has shown that the
resistivity of the samples is very high, like a dielectric material, the
transition of “metallic-semiconductor” conductivity behaviors is observed at
high temperature and the conductivity mechanism obey the nearestneighbour hoping (NNH) with the Arrhenius law and the variable-range
hopping (VRH) conductivity law.
Key words : Double rare-earth elements, resistivity, conductivity,
Arrhenius, VRH.
INTRODUCTION
Technologies for recycling waste heat are important for the efficient use
of energy. Thermoelectric materials are candidates for the recycling of waste
heat since they can directly convert heat into electric energy. Oxide ceramics
have received attention as thermoelectric materials since they are relatively
stable compared to inter-metallic materials in high temperature atmospheres.
Several perovskite-type oxides have been reported to show high
thermoelectric properties, such as the oxides CaMnO3-based, LaFeO3-based
[1,2]. Though the mentioned oxides have high Seebeck coefficient, the
electrical conductivity is low, it is necessary to improve the conductivity for
applications as thermoelectric materials. Generally, the thermoelectric
perovskite oxides are magnetic and semiconductor (n-type or p-type)
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behaviors. The electrical conduction mechanism is hopping process [3] . In
the present report, the electric property of the double rare-earth elements
perovskite compounds of (La1-xYx)FeO3 ( y = 0; 0,15, 0,25; 0,35; 0,45; 0,55;
1) have been investigated.
EXPERIMENTAL
(La1-xYx)FeO3 (x, y = 0; 0,15, 0,25; 0,35; 0,45; 0,55; 0; 1 ) were prepared
using the conventional solid-state synthesis technique. La2O3 ,Y2O3, and
Fe2O3 powders were used as starting materials. The mixed powder was
pressed into the pellets and pre-sintered in air at 10000C for 10 h. The
calcined pellets were ground again and pressed as tablets and finally sintered
at 12300C for 5 hrs.
The samples we examined under X-ray diffraction using D5005-Bruker
X-ray diffractometer and Cu-Kα radiation. The DC resistivity was measured
using a Keitheley 197A electrometer at various temperatures.
RESULTS AND DISCUSSION
The X-ray diffraction analysis of (La1-xYx)FeO3 indicates that all samples
possess an orthorhombic structure. The requirement for stability of this
orthorferrite is that the Goldschmidt tolerance factor t should be of nearly
unity, where t is defined via
RA+ R0 = t 2( RB  RO )
Where RA, RB and R0 are radii of A , B sites and oxygen ion respectively.
The lattice parameters (a, b, c), tolerance factor (t), and RA, RB radii of A
and B of the samles
(La1-xYx)FeO3 as function of Y content are listed in the table 1.
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Table 1. The lattice parameters, tolerance factor, and RA, RB radii as function
of Y content
X
Compound
a, A0
0
0.15
0.25
0.35
0.45
0.55
1
LaFeO3
La0.85Y0.15FeO3
La0.75Y0.25FeO3
La0.65Y0.35FeO3
La0.55Y0.45FeO3
La0.45Y0.55FeO3
YFeO3
5.56
5.54
5.54
5.55
5.55
5.55
5.28
b,
A0
5.55
5.57
5.53
5.52
5.47
5.45
5.59
c, A0 V,(A0)3
7.86
7.84
7.84
7.84
7.83
7.83
7.60
242.24
241.93
240.19
240.19
237.71
236.84
224.32
t
RA
A0
RB
A0
0.921
0.913
0.908
0.903
0.862
0.892
0.87
1.36
1.33
1.32
1.3
1.28
1.27
1.19
0.645
0.645
0.645
0.645
0.645
0.645
0.645
The volume of the unit cell is decreased with increasing the Y content
because the radius of Y3+ (0.097Ǻ) are smaller than La3+(0.114Ǻ).
In general, the samples of (La1-xYx)FeO3 are insulator. However, during
sintering at high temperature, some of Fe3+ transfer to Fe2+. Depending on
sintering temperature, cooling speed and atmosphere, the ratio of Fe3+//Fe2+ is
varied. The samples of (La1-xYx)FeO3 become conductor but their resistivity is
very high.
Fig.1 shows the ρ(T) dependences of (La1-xYx)FeO3 with x=0.15;
0.25(a); x=0.35; 0.45(b); x=0.55; 1(c) and x = 0(d). The small figures in the
top right corner are the decreasing branch of the ρ(T) curves.
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(a)
(c)
(b)
(d)
Fig.1 The ρ(T) dependence of (La1-xYx)FeO3
x = 0.15; 0.25 (a); x = 035; 0.45 (b); x = 0.55; 1.00(c); x = 0 (d)
It can be seen from these curves, the resistivity (ρ) is increased from room
temperature to around 350-400K (as metallic behavior) and then it is
decreased with increasing temperature (as semiconductor behavior). Around
between 300 and 450K, the resistivity of the samples with x=0.35; 0.45 is too
high to measure.
It is clear that, here, in the samples, there are two conductivity behaviors
of metal and semiconductor at different temperature ranges. In the work [1],
the electrical conductivity of LaFeO3 was not measured below 600K because
of the high electrical resistance of the sample. In our work, for the first time,
the transition of “metallic-semiconductor” conductivity behavior was
observed at the high temperature (300K-600K) in the very high resistivity
perovskite compound. From the lnρ(T) (Fig.2) of the decreasing branch of
ρ(T) curves (Fig.1), the electrical conduction mechanism in the samples can
be suggested as following.
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Here, we can apply two conductivity mechanisms : the nearest-neighbor
hopping (NNH) conductivity with the Arrhenius law
 (T )  0 exp  Ea / (kT )
and the variable-range hoping (VRH) conductivity satisfying a general law
 (T )  0 exp (T0 / T ) p 
The VRH conductivity is analysed within the Mott( p = ¼) or the
Shklovskii-Efros (SE, p = ½) concepts , depending on the importance of the
Coulomb interaction between the localized electrons [4]. Moreover, in [4] ,
the authors also give the different values of p : 3/4 ; 1/4 ; 1/3 or 1/2.
(a)
(b)
Fig.2 The lnρ vs. T curves of La1-xYxFeO3 , x = 0.15
Fig.2,3,4 show the lnρ(T) curves of the samples La 1-xYxFeO3 , x = 0.15;
0.25; 0.55, respectively.
In our cases, with x = 0.15 ( Fig.2), in the temperature range of (442K489K), the law lnρ ~ T-1 is satisfied (a) and in one of (533K-564K), the plot of
lnρ vs. T-3/4 can be fitted approximately with a linear law (b).
With x = 0.25 (Fig.3), in the range of (437K-455K) and (467K-500K) the
law lnρ ~T-1/4 (a) and lnρ ~T-3/4 (b) are fitted, respectively.
With x = 0.35; 0.45; 0.55 (Fig.4), in the temperature range of ( 437K553K), the law lnρ ~T-1/2 (a); lnρ ~T-1/4 (b) and lnρ ~T-1 (c) are satisfied ,
respectively.
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(a)
(b)
Fig.3 The lnρ vs. T curves of La1-xYxFeO3 , x = 0.25
Fig.4 The lnρ vs. T curve of La1-xYxFeO3
x = 0.55
CONCLUSION
The double rare-earth elements perovskite compound of (La1-xYx)FeO3 (x
= 0; 0,15, 0,25; 0,35; 0,45; 0,55; 0 ; 1 ) were prepared successfully by ceramic
method. The samples are single phase of orthorhombic structure. The volume
of unit cell is decreased with increasing of Y content. For the first time, the
transition of “metallic-semiconductor” conductivity behaviors is observed in
the doped perovskite having high resistivity like a dielectric material at high
temperature ( >300K ). The samples are semiconductor at the temperature
higher about 400K and the conductiviy mechanism obey the Arrhenius law (
the nearest-neighbor hopping-NNH) and the variable-range hopping (VRH)
law.
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Acknowledgment
The authors would like to thank “National Foundation for Science and
Technology (NAFOSTED)”, with the project code “103.03.69.09”, for
financial support.
References
1
Kouta Iwasaki, Tsuyoshi Ito, Masahito Yoshino, Tsuneo Matsui,
Takanori Nagasaki, Yuji Arita Journal of Alloys and Compounds 430
(2007) 297-301.
2.
S. Komine, E. Iguchi Journal of Physics and Chemistry of Solids 68
(2007) 1504-1507.
3.
Ming-Hao Hung, M.V Madhava Rao, Dah-Shyang Tsai Materials
Chemistry and Physics 101 (2007) 297-302.
4.
K.G Lisunov, E.A Arushanov, B. Raquest, J M Broto, F C. Chou, N.
Wizent and G. Behr J.Phys.
Condens. Matter 18 (2006) 8541-8549.