CC BY
98
yflK 54.057
RELAXOR PROPERTIES OF CERAMIC SOLID SOLUTIONS IN SrTiO3-BiScO3 SYSTEM E.P. Danshina, O N. Ivanov, T.I. Yurchenko
Belgorod State University,
Pobedy St., 85, Belgorod, 308015, Russia, e-mail: Ivanov.Oleg@bsu.edu.ru
Abstract. For the first time, ceramic solid solutions of (1-x)SrTiO3-x BiScO3 system with x = 0,0.05, 0.1, 0.2, 0.25,0.3,0.35, 0.4, 0.45 and 0.5 have been synthesized via solid-state processing techniques. End compounds of this system are not ferroelectric materials. TheXRD analysis revealed that, at room temperature, all compositions under study (excepting pure SrTiO3) consist of mixture of the center-symmetric cubic Pm3m phase and the polar tetragonal P4mm phase. The EBSD mapping has applied to image two-phase structure. It was found that fraction of the cubic phase decreases and fraction of the tetragonal phase increases when x arises. Anomalous behavior of dielectric permittivity and dielectric losses have been found for these samples which are specific for ferroelectrics with diffuse phase transition.
Key words: relaxors, diffuse phase transition, dielectric properties.
1. Introduction
Relaxor ferroelectrics may be considered as natural nanocomposites consisting of polar nanoclusters distributed inside nonpolar matrix [1]. It was recently found that SrTiO3-BiScO3 system is new system with relaxor properties [2, 3]. In this system, strontium titanate, SrTiO3, is known to be an incipient ferroelectric lying near the limit of its paraelectric phase stability [4]. Various ions substituted for the host ions both in the A- and B- positions in the ABO3 perovskite structure can induce the ferroelectric phase transition in SrTiO3.
BiScO3 is an interesting end member for fabrication of new ceramic solid solutions [5]. Despite its utility in solid solutions, there is a little knowledge about the BiScO3 member itself. In particular, it is not known at present is whether BiScO3ferroelectric [6]? Although it has been speculated that it may be ferroelectric, no experimental confirmations were reported.
The SrTiO3-BiScO3 system is the new and attractive one in the family of ceramic solid solutions with BiScO3 as one of the end members. At room temperature, SrTiO3 has the cubic Pm3m structure while BiScO3 is the monoclinic C2/c compound. Therefore, change of symmetry from cubic to monoclinic should be observed as the mole fraction of BiScO3 increased. Moreover, taking into account the essential difference in structures of end members in the SrTiO3-BiScO3 system, intermediate phases with other symmetries including polar structures may be expected to be formed for some compositions.
2. Experimental procedure
Ceramic solid solutions of (1-x)SrTiO3-x BiScO3 with x =0, 0.05, 0.1, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45 and 0.5 were synthesized via solid-state processing techniques from powders of SrCO3, TiO2, Bi2O3 and Sc2O3 taken as starting materials. After preliminary milling and drying, powders were calcined at 1073 K for 4 h and at 1123 K for 4 h at air atmosphere. Then, calcined powders were cold isostatically pressed at 400 MPa. The pressed samples were sintered at 1623 K for 5 h. The weight loss during sintering was confirmed to be < 1% for all samples. The additional 3 mol% Bi2O3 was added as sintering aid before pressing the samples to compositions with x =0, 0.05 and 0.1. It is known that the excess Bi2O3 can improve the densities of samples during the sintering because Bi2O3 has the melting temperature near 1100 K which is lower than the sintering temperatures used in this study. The densities of all samples were higher than 90% value of theoretical density.
The X-ray diffraction analysis (XRD) was performed at room temperature for phase determi-nation using the Rigaku Ultima IV diffractometer with CuKa radiation. The EBSD method (scanning electron microscope Quanta 200 3D) was used to estimate the distribution of phases with various crystal structures. Dielectric properties of samples were measured using the BR2876 LRC-meter.
3. Experimental results and discussion
Analysis of the XRD patterns taken at room temperature allows to conclude that all compositions under study (excepting pure SrTiO3) consist of mixture of nonpolar cubic Pm3m phase and polar tetragonal P4mm phase. Concentration dependences of lattice parameters for tetragonal, aT and bT, and cubic, aC, phases extracted from the XRD patterns are presented in Fig. 1 (a) and (b). Tetragonal structures can be formally reduced to cubic structures with unit-cell parameter calculated as ar = (a2 bT )1/3 where aT and bT are tetragonal cell parameters derived experimentally. The ar(x) 'dependence is also shown in Fig. 1 (c). All lattice parameters in Fig. 1 arise with the x increasing. However, in contrast with Vegard’s law prediction all concentration dependences of lattice parameters are nonlinear.
In order to get an additional evidence of two-phase coexisting in samples under study, the EBSD mapping was applied. Imaging of a few phases with various structures can be done by this method. As an example, Fig. 3 shows EBSD images for samples with x=0.2 and 0.45 taken at room temperature. In this figure, the red color corresponds to cubic phase while the green color presents tetragonal phase. One can see that the cubic phase is predominant for the composition with x = 0.2, but the composition with x = 0.45 is enriched by the tetragonal phase. The dependances of cubic and tetragonal phase fractions, Vr are presented in Fig. 4. It was found that the fraction of cubic phase decreases and the fraction of tetragonal phase increases when x arises.
It is known [1] that the phase coexisting of polar and nonpolar phases at some temperature range is one of specific signs of relaxor ferroelectrics. It is important that almost all of models of relaxor state consider relaxor as a nonpolar paraelectric matrix containing the polar nanoregions. Each of these nanoregions may have its own local Curie temperature. The
polar nanoregions with the suitable Curie temperatures will take part at phase transition at cooling from paraelectric phase to relaxor state consisting of polar nanoregions distributed within a nonpolar matrix. So, the diffusing of ferroelectric phase transition is originated from distribution of Curie's temperatures in inhomogeneous material.
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a;
m
c)
d)
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2 3 4
Fig. 1. Concentration dependences of lattice parameters for various phases of ceramic (1-x)SrTiO3-xBiScO3 materials.
Fig. 2. Dependences of ln[(em/e ) - 1] versus ln(T - Tm) for samples with x =0.2 (a), 0.25(b), 0.3(c), 0.4(d) and 0.45 (e).
Relaxors are characterized by anomalous behavior of dielectric properties. Fig. 5 shows the dielectric permittivity e and e versus temperature of samples with x=0.2, 0.25, 0.3, 0.4 and 0.45 (the measurement frequency is 1 MHz). Broad peaks of e are observed in the e (T) dependences. It was found that the temperature Tm of the e (T) peak increases with the
increasing of BiScO3 content. In addition, the increasing of dielectric losses e was observed at cooling of samples below Tm. Such a kind of anomalous behavior of dielectric properties is typical for ferroelectrics at ferroelectric phase transitions.
Forferroelectrics with sharp phase transition the peak of the e (T) temperature dependence in high-temperature region obeys the Curie-Weiss law. In this case the dependence of 1/e versus temperature should be linear. It was found that experimental e (T) curves in Fig. 5 start to deviate from the Curie-Weiss behavior just below some temperature Td [7]. This feature can be taken as an evidence of the diffusing of phase transition under study. The temperature Td called as the Burns temperature is corresponding to appearance of polar nanoregions inside nonpolar matrix during diffuse phase transition.
a)
b)
Fig. 3. EBSD images of samples with x = 0.2(a) and 0.45 (b).
0.2
0.4
0
%
Fig. 4. Concentration dependences of the cubic and tetragonal fractions of ceramic (1-x)SrTiO3-xBiScO3 materials.
Temperatures Tm and Td, and the Curie temperature 0 and also the Curie-Weiss constant CCW of samples (1-x )SrTiO3-x BiScO3 with various x are listed in Table 1.
200 400 600 T. K
100 200 300 400 T, K
Fig. 5. Temperature dependences of e (a) and e (b) corresponding to (1-x)SrTiO3-xBiScO3 solid solutions: 1 - x = 0.2,2 - 0.25, 3 - 0.3,4 - 0.4 and 5 - 0.45.
For diffuse phase transition, the e (T) dependence between Td and Tm can be fitted by expression [8]
em (T-Tm )Y
------= 1+----------------- (4.1)
e(T) 2c2 K '
where c is the diffuseness degree of phase transition and y is the dielectric relaxation degree.
Table 1
Dielectric data and characteristics of diffuse phase transition in (1-x)SrTiO3-x BiScO3
x Tm, K Td, K 0, K Ccw , 105, K
0.2 245 505 145 1.05
0.3 358 550 155 1.40
0.35 370 565 175 1.29
0.4 470 640 180 1.34
0.45 430 585 160 1.11
For sharp ferroelectric phase transition, y = 1 and its consequent diffusing leads to the y increasing up to 2 so that at larger y values it is appeared more relaxor behavior. By analyzing of dielectric properties between T d and Tm of samples with x=0.2, 0.25, 0.3, 0.4 and 0. 5, it was found that expression (1) reproduces experimental data very well (Fig. 2). Both y and ct were determined from linear parts of ln[(em/e )-1] versus ln(T - Tm) dependences.
Let us analyze the concentration dependence of ct within the framework of the Smolenskii-Isupov model [9] developed for two-component systems A(B1 _XB x)O3 or (A1 _xAx)BO3. This model gives the following expression for the diffuseness degree
ct = W-X) 1/2 (4.2)
n
where n is the number of unit cells in critical nucleus and 5 is the parameter characterizing the concentration dependence of Curie's temperature.
According to the expression (2), the diffuseness degree should be increased with the increase of disorder degree in the system under study. The disorder degree can be approximately expressed as the product of x(1 _ x). It was found that experimental dependence of ct versus x(1 _x) for samples understudy is in qualitative agreement with the expression (1) (Fig. 6). The higher diffuseness degree of samples with the greater percentage of BiScO3 can be explained by the increased cation disorder due to substitution on the A-site by Bi and on the B-site by Sc in the SrTiO3 structure.
a, i:
3 0 0 -
20 0 -
100 -
0,16 0.20 0.24
1(1 M
Fig. 6. Dependence of ct versus x(1 _ x) of ceramic (1 _ x)SrTiO3-xBiScO3 materials.
Temperature dependence of relaxor dielectric permittivity e can be analyzed in the spherical ’’random field-random bond” model [10]. The SRFRB model predicts the transition to ferroelectric phase, the dipolar glass state or mixed ferroelectric-dipolar glass state at various values of model parameters. One can find parameters of the SRFRB model using data of dielectric measurements.
The order parameter q of transition to dipolar glass state above Tm can be calculated from the corresponding £ (T) curve using the following relationship
^) = 1 -Ccw + (Jo/k)£(T) ■ (4 3)
Here, Jo is the mean energy of polar cluster interaction and k is the Boltzmann constant. If q = 0, then the expression (3) is transformed to the Curie-Weiss law where the temperature 0 = J0/k is introduced. As an example, the q temperature dependence of sample with x = 0.2 is shown as the scattered curve in Fig. 7.
At the same time, the parameter q can be expressed by microscopic model parameters
( A 2 2
q = P2J2 q + J2 (1 -q)2 + P 2 (4.4)
where P =1/kT , P is the polarization, A and J 2 are variances of the random field strength and the interaction energy, correspondingly. The q(T) dependence calculated using the formula (4) at temperatures T > Tm where P = 0, is satisfactory agreed with experimental q(T) curve (solid line in Fig. 7).
t, SS
Fig. 7. The q(T) dependence for the sample with x=0.2. Solid line is the SRFRB model curve.
The SRFRB model parameters extracted from the best fitting for all compositions under dielectric measurements are listed in Table 2. The SRFRB model predicts the transition to spherical glass state when the following condition is fulfilled
8 ( A
Jo = 0 < J 2+ A 2. (4.5)
k k k2
Taking into account the expression (5) one can conclude from data of Tables 1 and 2 that samples of the (1-x)SrTiO3-x BiScO3 system should undergo the transition to spherical glass state.
Table 2
SFRRB model parameters
x A/J2, K-2 J /k, K A/k2, K2
0.2 0.004 410 670
0.3 0.002 470 440
0.35 0.0008 480 185
0.4 0.0006 590 210
0.45 0.0006 545 180
X-ray data and results of the EBSD mapping can be taken as evidences of glass state in samples understudy. Nonpolar cubic phase can be attributed to this state. However, it has been found that the polar tetragonal phase may co-exist with the cubic phase.
So, the mixed ferroelectric-dipolar glass state can be realized in the ceramic of (1-x )SrTiO3-x BiScO3 materials.
It is known [11] that simultaneous substitution of host Sr2+ and Ti4+ ions in the SrTiO3 structure as impurity ions at the production of solid solutions leads to some specific effects. For example, the diffuse ferroelectric phase transition in SrTiO3-PbMg1 /3Nb2/3O3 was observed only at x > 0.2 with the linear dependence of transition temperature on the composition which may be associated with random fields due to the disordered Mg2+ and Nb5+ distribution.
Property peculiarities of the SrTiO3-BiScO3 system having been found in our research are similar to the same peculiarities of the SrTiO3-PbMg1/3Nb2/3O3 system and they can be attributed to the diffuse ferroelectric phase transition. But in contrast with the SrTiO3-PbMg1/3Nb2/3O3 system where PbMg1/3 Nb2/3O3 is relaxor ferroelectric, in SrTiO3-BiScO3 both of the end members are not ferroelectric.
3. Conclusion
Ceramic solid solutions of the (1-x)SrTiO3-x BiScO3 system with x=0, 0.05,0.1,0.2,
0.25, 0.3, 0.35, 0.4, 0.45 and 0.5 were synthesized via solid-state processing techniques. End members in this system are not ferroelectric. The XRD analysis revealed that, at room temperature, all compositions understudy (excepting the pure SrTiO3) consist of mixture of the nonpolar cubic Pm3m phase and the polar tetragonal P4mm phase. The EBSD mapping was applied to image the two-phase structure. It was found that fraction of the cubic phase decreases and fraction of the tetragonal phase increases when x arises.
Dielectric anomalies associated with diffuse ferroelectric phase transition were found and analyzed for compositions pointed out.
Due to complex processes of cation substitution and ordering on the A-site and on the B-site, the relaxor ferroelectric behavior was like.
Acknowledgeme
nts
The work was performed in the framework of the federal target program “Research and
Development on Priority Directions of Scientific-Technological Complex of Russia in
2007—
2012” under Contract No 16.552.11.7004.
^MTepaTy
pa
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РЕЛАКСОРНЫЕ СВОЙСТВА КЕРАМИЧЕСКИХ ТВЕРДЫХ
РАСТВОРОВ СИСТЕМЫ SrTiOs-BiScOa
Е.П. Даньшина, О.Н. Иванов, Т.И.
Юрченко
Белгородский государственный университет,
ул. Победы, 85, Белгород, 308015, Россия, e-mail: Ivanov.Oleg@bsu.edu.ru
Аннотация. Впервые синтезированы керамические твердые растворы системы (1-x )SrTiO3- xBiScO3 с x = 0, 0.05,0.1, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45 и 0.5 по керамической технологии. В данной системе оба краевых компонента не являются сегнетоэлектрическими материалами. Данные рентгеноструктурного анализа показали, что при комнатной температуре все соста- вы (кроме чистого SrTiO3) состоят из смеси центро-симметричной кубической Pm3m фазы и полярной тетрагональной P4mm фазы. Проведен EBSD анализдля подтверждения наличия двухфазной структуры. Было обнаружено, что количество кубической фазы уменьшается, а тетрагональной фазы
увеличивается при увеличении концентрации х.
Для этих образцов на температурных зависимостях диэлектрической проницаемости и диэлектрических потерь обнаружены аномалии, характерные для сегнетоэлектриков с размытым
фазовым переходом.
Ключевые слова: релаксоры, размытый фазовый переход, диэлектрические свойства.
