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УДК 54.057, 537.9
COEXISTING OF CUBIC AND TETRAGONAL PHASES IN PEROVSKITE-TYPE SrTiOs - BiScOs RELAXOR SYSTEM
E.P. Danshina, O.N. Ivanov, D.A. Kolesnikov
Joint Research Centre «Diagnostics of Structure and Properties of Nanomaterials», Belgorod State National Research University, Belgorod;
Danshina@bsu.edu.ru
Recommended for Publication by Editorial Member Professor N. Ts. Gatapova
Kew words and phrases: crystal structure; phase coexisting; relaxor ferroelectrics.
Abstract: Dielectric anomalies specific for to relaxor ferroelectrics have been found for the SrTiO3 - BiScO3 system. The relaxor properties of this system are rather unexpected because neither SrTiO3 nor BiScO3 are ferroelectrics. An X-ray diffraction analysis revealed that at room temperature the ceramic (1 - x)SrTiO3 - xBiScO3 samples with x = 0.2, 0.3 and 0.4 consist of a mixture of a cubic centrosymmetric Pm3m phase and a tetragonal polar P4mm phase. Lattice parameters for these phases increase as x increases. In addition, it was found by the electron backscattered diffraction method that at room temperature, the fraction of the cubic phase decreases and the fraction of the tetragonal phase increases as the mole fraction of BiScO3 increases. Phase coexistence is assumed to be one of the reasons for the relaxor properties of the system under study.
Perovskite-type SrTiO3 - BiScO3 system is a new system enabling to study the possibility of ferroelectric state formation in multicomponent systems consisting of non-ferroelectric components.
In this system strontium titanate, SrTiO3, is known to be an incipient ferroelectric lying near the limit of its paraelectric phase stability [1]. Pure SrTiO3 retains a nonpolar centrosymmetric crystal on cooling down to the lowest temperature at T ^ 0. Weak external influences including various impurities substituted for the host ions in crystal structure can destroy a paraelectric state and induce a ferroelectric phase transition in incipient ferroelectrics [2-7].
BiScO3 is an interesting end member for fabrication of new ceramic solid solutions [8-10]. Despite its utility in solid solutions, there is little knowledge about BiScO3 member itself. Although it has been speculated that it may be ferroelectric, no experimental confirmations have been reported [11].
The factors favouring the formation of the ferroelectric state in the SrTiO3 - BiScO3 system consisting of non-ferroelectric end members are as follows:
(i) The crystal symmetries of end members are significantly different. At room temperature SrTiO3 has a cubic Pm3m structure, while BiScO3 is a non-polar monoclinic C2/c compound [12]. So, formation of some intermediate phases with
symmetries other than cubic SrTiO3 symmetry and monoclinic BiScO3 symmetry can be assumed for SrTiO3 - BiScO3 system. Such intermediate phases including possibly having polar structures are necessary to ensure a significant change in the crystal structure of the system under consideration as the mole fraction of BiScO3 increases.
(ii) As it was mentioned above SrTiO3 is one of incipient ferroelectrics. Therefore, one can assume that the substitution on the A-site by Bi and on the B-site by Sc in the SrTiO3 structure will destroy a paraelectric state in the SrTiO3 - BiScO3 system.
In fact, it was recently found that the SrTiO3 - BiScO3 system is characterized by dielectric anomalies specific for relaxor ferroelectrics or ferroelectrics with diffuse phase transition [13, 14]. The main features of relaxors are connected with their structural (compositional) inhomogeniety [15-17]. In terms of structural features the relaxor properties can be attributed to coexistence and interaction of polar and non-polar phases in the temperature range of relaxor state existence. Preliminary results have allowed us to conclude that coexistence of polar tetragonal P4mm phase and non-polar cubic Pm3m phase can lead to appearance of relaxor properties of the SrTiO3 - BiScO3 system [13-14].
The main purpose of this paper is to characterize further two-phase state in ceramic samples of the SrTiO3-BiScO3 relaxor system.
Ceramic samples of (1 - x)SrTiO3 - xBiScO3 with x = 0.2, 0.3 and 0.4 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 in an air atmosphere.
The calcined powders were then 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 densities of all samples were higher than 90 % of the value of the theoretical density.
X-ray diffraction (XRD) analysis was performed at room temperature for phase composition and crystal structure determination using a Rigaku Ultima IV diffractometer with CuKa radiation (a step width of 0.02o and a counting time of 1 s/step). A scanning electron microscope, the Quanta 200 3D, was used to apply the electron backscattered diffraction (EBSD) method to estimate the distribution of tetragonal and cubic phases in the samples under study (for an accelerating voltage of 20 kV and a typical current of 12 nA).
The dielectric permittivity s was measured using a BR2876 LRC-meter at a frequency of 1 MHz.
Before to characterize the features of two-phase state, let us consider the dielectric properties of the compositions under study.
Figure 1 shows the dielectric permittivity versus temperature for the samples with x = 0.2, 0.3 and 0.4. Broad peaks of e are observed in the T-dependences for these compositions. It was found that maximum value of dielectric permittivity, em, and temperature of the e(T) maximum, Tm, increase as x increases.
e
T, K
Fig. 1. Temperature dependences of £ for the (1 -x)SrTiO3 - xBiScO3 samples:
1 - x = 0.2; 2 - 0.3; 3 - 0.4
1/e, 10-
For ferroelectrics with a sharp phase transition, the temperature dependence of s for the high-temperature part of the e(T) peak obeys the Curie-Weiss law
e =
cCW T - T
(1)
where CCW is the Curie-Weiss constant and the temperature Tm is coincident with the Curie temperature. In this case the dependence of 1/e versus temperature (or the temperature difference (T - Tm)) should be linear. Figure 2 shows that
experimental e(T) curves are linear above some temperature Td. Just below Td experimental curves start to deviate from the Curie-Weiss behavior.
A broad maximum in the e(T) dependence with the characteristic
temperature Td can be originated from a diffuse phase transition. The temperature Fig. 2. Temperature dependences of 1/s Td, called the Burns temperature,
for of (1 -x)SrTiO3-xBiScO3 samples: corresponds to the appearance of the polar
1 - x = 0.2; 2 - 0.3; 3 - 0.4 phase during the diffuse ferroelectric
phase transition [18].
The temperatures Tm and Td extracted from the e(T) dependences are listed in Table. According to Table, both temperatures are shifted to the high temperature range as x increases.
Because the temperature Td is higher than room temperature, TR, all of the compositions under investigation at TR are in the relaxor state in which the polar and nonpolar phases coexist. The characteristics of the crystal lattice for these phases and the phase distribution should be x-dependent.
XRD analysis was applied to characterize the crystal structure features for sintered (1 - x)SrTiO3 - xBiScO3 samples. The XRD patterns taken at TR are shown in Fig. 3. The XRD pattern for pure SrTiO3 is also presented in this figure. The compositions with x = 0.2, 0.3 and 0.4 consist of a mixture of the cubic Pm3m phase and the tetragonal P4mm phase, while the composition with x = 0 has cubic Pm3m symmetry.
Characteristics of (1 - x)SrTiO3 - xBiScO3 samples
x Tm, K Td, K Lattice parameter for the cubic phase, ac, A Lattice parameters for the tetragonal phase Tetragonality, cT/aT Tetragonal phase fraction
A ct, A
0.20 245 505 3.908 3.916 3.923 1.0018 0.25
0.30 358 550 3.931 3.921 3.930 1.0023 0.43
0.40 470 640 3.937 3.932 3.948 1.0041 0.67
Intensity
101/110
20000 counts
30 40 50 60 70
20, degrees
Fig. 3. X-ray diffraction patterns of the (1 -x)SrTiO3 -xBiScO3 samples:
♦ - Pm3m; ■ - P4mm
Intensity
20, degrees
a)
Intensity
20, degrees
b)
Fig. 4. Enlarged part of the diffraction peak in the range of20 = 51.2 - 53.5 for the sample with x = 0 (a) and x = 0.3 (b):
C - cubic phase; T - tetragonal phase
The tetragonal P4mm structure is characterized by splitting of the single cubic (200) peak into double diffraction (002)/(200) peaks, as is shown in Fig. 4.
Additional right-side peaks in Fig. 4 are due to the Cu^a2 radiation. The methods of Savitzky-Golay [19] and Sonnevtld-Visser [20] were applied to analyze the XRD patterns. The grey lines in Fig. 4 give the diffraction peaks calculated using this
analysis. The lattice parameters were determined from at least six or four indexed
diffraction peaks for the tetragonal (aT and bT) and cubic (ac) phases, respectively.
The concentration dependencies of the lattice parameters for these phases extracted
from the XRD patterns are listed in Table. All lattice parameters in Table increase as x
increases. Such behaviour is predicted when taking into account the difference in radii
for pairs of ions at equivalent sites in the perovskite ABO3 lattice (r(Sr2+) = 1.12 A,
r(Bi3+) = 1.34 A for the .4-sublattice and r(Ti4+) = 0.745 A and r(Sc3+) = 0.885 A for
the 5-sublattice). Because of the significant difference in the ionic radii, r(Sr2+)/r(Bi3+) = 4+ 3+
= 0.836, and r(Ti )/r(Sc ) = 0.842, the unit cell volume drastically increases as the mole fraction of BiScO3 increases. It should also be noted that the tetragonality degree, cT/aT, is small and that it gradually increases as x increases.
To obtain additional evidence of the coexistence of two phases in the samples of the (1 - x)SrTiO3 - xBiScO3 system, the EBSD method was applied (Fig. 5).
Based on the symmetries of the tetragonal and cubic phases determined from XRD analysis, the phase distribution can be mapped by this method. The top images in Figure 5 are EBSD inverse-pole-figure maps for the cubic phase (black domains are the tetragonal phase), and the middle images are EBSD inverse-pole-figure maps for the tetragonal phase (black domains correspond to the cubic phase). The bottom images in Fig. 5 show EBSD phase distribution maps for the samples with x = 0.2, 0.3 and 0.4 taken at room temperature. In this figure, the red colour corresponds to the cubic phase, while the green colour represents the tetragonal phase.
a) b) c)
Fig. 5. EBSD maps for the samples with x = 0.2 (a), 0.3 (b) and 0.40 (c).
The top images are EBSD inverse-pole-figure maps for the cubic phase, the middle images are EBSD inverse-pole-figure maps for the tetragonal phase and the bottom images are EBSD phase distribution maps
One can observe that the fraction of the cubic phase decreases and the fraction of the tetragonal phase increases as x increases. For all compositions, the tetragonal phase is presented by sufficiently large domains whose sizes consistently increased from 2-3 ^m for x = 0.2 to 5-7 ^m for x = 0.4. In addition, there are many smaller tetragonal islands with size < 1 ^m in the sample with x = 0.2. Such islands are practically absent for the other compositions.
It is known for relaxors that upon cooling below Td, small polar nanodomains appear whose growth and interactions can induce a transition from the relaxor state into a glassy or ordered phase [21]. If the nanodomains grow but do not become large enough, they will ultimately demonstrate a dynamic slowing down of their fluctuations at cooling below Tm, leading to an isotropic relaxor state with random orientation of the polar domains. If the domains become large enough, the relaxor sample will undergo a cooperative ferroelectric phase transition below Tm. Thus, a transition from the relaxor state to the ferroelectric state can be assumed for some relaxors.
Figure 5 shows that the polar tetragonal phase is represented by large domains and that the fraction of this phase increase when the Burns temperature shifts to the high-temperature range. This fact can be taken as evidence that for the system under study, the polar nanodomains appearing at Td have a tendency towards macroscopic growth and overlapping characteristics for the transition from the relaxor state to the ordered ferroelectric state.
Thus, it is found that the polar P4mm phase can be formed in the ceramic samples of the SrTiO3 - BiScO3 system consisting of end non-polar members. The tetragonal phase coexists with the cubic centrosymmetric Pm3m phase. At room temperature, the fraction of the cubic phase decreases and the fraction of the tetragonal phase increases when x increases. Phase coexistence is assumed to be one of the reasons for the relaxor properties for the system under study.
This work has been financed by the Ministry of Education and Science of the Russian Federation under Contracts No. 16.552.11.7004 and No. 14418.21.1155.
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Сосуществование кубической и тетрагональной фаз в релаксорной системе 8гТЮз - Б18еОз перовскитового типа
Е.П. Даньшина, О.Н. Иванов, Д.А. Колесников
Центр коллективного пользования научным оборудованием «Диагностика структуры и свойств наноматериалов»,
ФГАОУ ВПО «Белгородский государственный национальный исследовательский университет» (НИУ «БелГУ»), г. Белгород;
Danshina@bsu.edu.ru
Ключевые слова и фразы: кристаллическая структура; релаксорные сегнетоэлектрики; сосуществование фаз.
Аннотация: Диэлектрические аномалии, характерные для релаксорных сег-нетоэлектриков, обнаружены при изучении системы БгТЮз - Б18е03. Релаксорные свойства этой системы являются достаточно неожиданными, так как ни БгТЮз, ни Б18е0з не являются сегнетоэлектриками. Рентгеновский анализ обнаружил, что при комнатной температуре керамические образцы (1 - х)БгТЮз - хБ18е0з с х = 0.2, 0.з и 0.4 состоят из смеси кубической центросимметричной Ртзт фазы и тетрагональной полярной Р4тт фазы. Параметры решеток этих фаз возрастают
при увеличении х. Кроме того, с помощью метода дифракции обратно рассеянных электронов, обнаружено, что при комнатной температуре доля кубической фазы уменьшается, а доля тетрагональной фазы возрастает при увеличении мольной доли Б18е0з. Предполагается, что сосуществование фаз является одной из причин появления релаксорных свойств исследуемой системы.
Koexistenz der kubischen und tetragonalen Phasen im Relaxorsystem SrTiO3 - BiScO3 des Perowskittypus
Zusammenfassung: Die dielektrischen Anomalien, die fur die
Relaxorferroelektriken charakteristisch sind, wurden bei dem Erlernen des Systems SrTiO3- BiScO3 nachgewiesen. Die Relaxoreigenschaften dieses Systems sind genug unerwartet, weil weder SrTiO3 noch BiScO3 keine Ferroelektriken sind. Die Rontgenanalyse hat gezeigt, dass bei der Zimmertemperatur die keramischen Muster (1 - x)SrTiO3 - xBiScO3 с x = 0.2, 0.3 und 0.4 aus dem Gemisch der kubischen zentrosymmetrischen Pm3m Phase und der tetragonalen polaren P4mm Phase bestehen. Die Parameter der Gitter dieser Phasen steigern bei der Vergrosserung von x. Ausserdem wurde es mit Hilfe der Methode der Diffraktion der indirect diffusen Elektronen entdeckt, dass bei der Zimmertemperatut der Anteil der kubischen Phase kleiner wird, und der Anteil der tetragonelen Phase bei der Vergrosserung des Molanteiles BiScO3 grosser wird. Es wird angenommen, dass die Koexistenz der Phasen einer der Grunde des Erscheinens der Relaxoreigenschaften des untersuchenden Systems ist.
Coexistance des phases cubique et tetragonale dans un systeme de relaxion SrTiO3 - BiScO3 du groupe de la perovskite
Resume: Les anomalies dielectriques typique pour les ferroelectriques de relaxion sont revelees lors de l’etude du systeme SrTiO3 - BiScO3. Les proprietes de relaxion de ce systeme sont assez inattendues puisque ni SrTiO3, ni BiScO3 ne sont pas ferroelectriques. L’analyse radiologique a montre que lors de la temperature de chambre les echantillons ceramiques (1 - x)SrTiO3 - xBiScO3 с x = 0.2, 0.3 et 0.4 se composent du melange centrosymmetrique cubique de la phase Pm3m et de la phase tetragonale polaire Pm4m. Les parametres des reseaux de ces phases augmentent lors de la croissance x. Outre cela a l’aide de la methode de la diffraction des electrons disperses inversement il est revele que lors de la temperature de chambre la part de la phase cubique dimunie et celle de la phase tetragonale augmente avec l’ augmentation de la part modulaire BiScO3. Est suppose que la coexistance des phases est une des raisons de l’apparition des proprietes de relaxion du systeme etudie.
Авторы: Даньшина Елена Павловна - кандидат физико-математических наук, научный сотрудник Центра коллективного пользования «Диагностика структуры и свойств наноматериалов»; Иванов Олег Николаевич - доктор физико-математических наук, директор Центра коллективного пользования «Диагностика структуры и свойств наноматериалов»; Колесников Дмитрий Александрович - заведующий лабораторией микроскопии и рентгеноструктурного анализа Центра коллективного пользования «Диагностика структуры и свойств наноматериалов», ФГАОУ ВПО «Белгородский государственный национальный исследовательский университет» (НИУ «БелГУ»), г. Белгород.
Рецензент: Красильников Владимир Владимирович - доктор физико-математических наук, профессор кафедры «Материаловедение и нанотехнологии», ФГАОУ ВПО «Белгородский государственный национальный исследовательский университет» (НИУ «БелГУ»), г. Белгород.
