ISSN 1062-8738, Bulletin of the Russian Academy of Sciences: Physics, 2016, Vol. 80, No. 11, pp. 1364–1366. © Allerton Press, Inc., 2016. Original Russian Text © S.V. Khasbulatov, L.A. Shilkina, H.A. Sadykov, A.A. Pavelko, A.T. Kozakov, S.P. Kubrin, I.N. Andryushina, L.A. Reznichenko, 2016, published in Izvestiya Rossiiskoi Akademii Nauk, Seriya Fizicheskaya, 2016, Vol. 80, No. 11, pp. 1539–1541.
Phase Formation and the Formation of Microstructures and Macroscopic Responses in BST Ceramics S. V. Khasbulatov*, L. A. Shilkina, H. A. Sadykov, A. A. Pavelko, A. T. Kozakov, S. P. Kubrin, I. N. Andryushina, and L. A. Reznichenko Research Institute of Physics, Southern Federal University, Rostov-on-Don, 344090 Russia *e-mail:
[email protected] Abstract⎯Patterns of phase formation and the formation of microstructure and macroscopic responses in (1 – х)BaTiO3–xSrTiO3 (0.00 ≤ х ≤ 1.00) ceramics are established. DOI: 10.3103/S1062873816110216
INTRODUCTION Binary system (1 – х)BaTiO3–хSrTiO3 (BST) is an example of ferroelectric (FE) and solid solutions (SSes) that are of high technical and technological value due to their broad use in the new generation of control and measuring equipment (phase-shifting units, delay lines, resonators, filters, and amplifying technology) [1]. The phase diagram of the system, which contains a morphotropic range (MR) (the zone of a concentration tetragonal (Т)–cubic (C) transition) in a narrow composite field with its center at х ~ 0.30, makes it of deep fundamental importance [2, 3]. Scientific interest in this system has recently revived, due to its interpretation as an object of physical consideration. However, this applies only to the study of specific chemical compositions (e.g., with х = 0.20, 0.30, and 0.50) [1]. There have been almost no systematic, detailed, carefully plotted investigations of the behavior of such SSes in the range of solubility of components (0.00 ≤ x ≤ 1.00) under the impact of combined external actions. Bearing in mind that the BST system remains unique and of practical importance today, determining the principles of phase formation and the formation of the microstructure and macroscopic responses in BST ceramics is of some relevance. EXPERIMENTAL SS systems were prepared via solid-phase reactions with subsequent sintering using conventional ceramic technology. X-ray studies were performed with powder diffraction using a DRON-3 diffractometer (Bragg–Brentano focusing). Microstructure was determined using optical and electron microscopes on special polished microsections and chips of ceramics. Low-temperature (Т < 300 K) measurements were
made with a Wayhe Kerr 6500B impedance analyzer by cooling the samples in the chamber of a CCS-150 closed-cycle helium refrigeration cryostat. Studies at Т > 300 K were performed with a measuring bench that included an Agilent E4980A LCR-meter. RESULTS AND DISCUSSION X-ray phase analysis showed that all of our SSes had perovskite-type structure with no inclusions. Figure 1 presents the dependences of parameters а, с; experimental and theoretical cell volumes VT, VC, and Vtheor; half-widths of X-ray lines 111 and 200, B; and densities of samples ρmeas, ρX-ray, and ρrel on x ((1) aт, (2) cт, (3) Vт, (4) Vtheor, (5) cT/aT, (6) В111, (7) В200, (8) ρmeas, (9) ρX-ray, (10) ρrel, (11) аC, and (12) VC) along with the plotted phase boundaries. In contrast to the general PD system [2, 3], where a tetragonal (Т) → cubic (C) transition was observed at х ~ 0.30, we recorded a morphotropic range (MR) with coexisting Т and pseudocubic (Psc) phases at 0.20 < х < 0.40. We could not clearly determine the symmetry in the latter phase due to strong diffuse scattering (especially at large diffraction angles) and very slight distortion of the C cell. In the range of 0.60 < х < 0.80, there was a second МR where the Psc and C phases coexisted. It can be seen that dependences Vexp(х) and Vtheor(х) are virtually parallel, meaning the type of SS (substitution) remains the same over the range of concentrations. Considering the well-known sensitivity and, in some cases, critical character of the properties of mixed oxides with perovskite-type structure (including alkaline-earth titanates) to their thermodynamic prehistory, we established the conditions of preparation, which determine the situation with defects in
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ρ, g/cm3 6 9 8
Т + Psc Т
ρrel, % 92 C
Psc + C Psc
400
Ti C(m3m)
10
I 76 B, ang. deg
7
4 (с/a – 1) 103 6 9 5 4 3 3 a, c, Å 2 4.01 1
1365
x = 0.3 1
0.3 0.2 V, Å3 66
T(4mm)
II 2
200 O(4mm) x = 0.7
3 R(3m)
III
63 12
60
11
0
0.4
0.8
x
3.92 0
0.2
0.4
0.6
0.8
1.0 x
Fig. 1. Dependences of parameters а, с; experimental and theoretical cell volumes VT, VC, and Vtheor of (1 – х)BaTiO3–xSrTiO3 solid solutions; half-widths of X-ray lines 111 and 200, B; and densities ρ of ceramic samples on x: (1) аT, (2) сT, (3) VT, (4) Vtheor, (5) сT/аT, (6) B111, (7) B200, (8) ρmeas, (9) ρX-ray, (10) ρrel, (11) аC, and (12) VC.
objects, patterns of the effect sintering temperature Тsin and the topology of samples have on the microstructure (grain structure) of ceramics, which largely determines their macroproperties. As our object of study, we chose an SS of Ba0.80Sr0.20TiO3 composition, prepared by varying Тsin from 1300 to 1530°C. The topology of ceramic samples was discs with ∅12 × 1– 4 mm, columns with ∅12 × 8 mm, and large blocks with ∅40–50 × 4–5 mm. We determined the critical dependence of mean grain size d of ceramics (discs) with two maxima at 1350 and 1500°C; the crust-layer microstructural type of large blocks with the development of secondary intermittent recrystallization and, as a consequence, the formation of bulky idiomorphic grains; microstructural heterogeneity with sections of ultrafine grains and ones an order of magnitude larger (columns); and the emergence of dark spots on the samples’ surfaces and dark veins in their bulk. These observations are associated with the redox processes of Ti4+ ↔ Ti3+ and the emergence, accumulation, ordering, and annihilation of oxygen vacancies via crystal-
Fig. 2. Dependences of the phase transition temperatures of the studied solutions on the concentration of SrxTiO3 (х).
lographic shear; the propagation of internal stresses with the growth of spontaneous deformation; the formation of liquid-like states due to the melting of Ba(OH)2, a product of the hydrolysis of the unreacted initial component BaCO3; and chemical reactions on the surfaces and in the bulk of samples that lead to the formation of carbon films. It was therefore shown that the grain structure of BST ceramics is remarkable and depends nonmonotonously on the Тsin of the samples and their topology. Patterns of the formation of dielectric responses of the system’s ceramics were established over a broad range of concentrations (0.00 ≤ х ≤ 1.00; ∆х = 0.10), temperatures (10–800 K); frequencies (f); and the variable measuring electric field (0.1–1000 kHz). It was found (Fig. 2) that substituting Sr for Ba leads to a linear reduction (with various rates in three ranges of concentration, I–III) in the temperatures of all three phase transitions (PTs) characteristic of BaTiO3; their approach and merging at х ~ 0.85; the diffusion of PTs at Т1; the emergence of dielectric dispersion around Т1 in an SS when 0.10 ≤ х ≤ 0.30 (it is especially strong in an SS when х = 0.10 and 0.0); a shift of the range of abnormal growth of ε/ε0 (a characteristic of pure BaTiO3) from Т ~ 500 K to Т1 in these SSes; the emergence of strongly relaxing maxima of ε/ε0 in the paraelectric (Pe) phase of an SS when х = 0.10 (dashed lines); their gradual shift to Т1 in SSes when х = 0.20; the convergence with Т1 in SSes when х = 0.30, and the development of a second relaxation process; and a reduc-
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tion in the hysteresis of ε/ε0 at the Curie point at Т1. These observations are associated with the change in the force constants of Ti–O and, as a consequence, the conditions of the hybridization of the electron states of Ti(3d) and O(2p) representing the key mechanism of ferroelectricity (FE), due to the reduction in the Тi–O–Ti bond angle upon replacing the bulky Ba ions (1.38 Å) with smaller Sr ions (1.20 Å), with oxygen ions being displaced closer to the Sr ions (a reduction in Т1 [1]); the formation of two MRs on the phase diagram of the system (a rapid change in Т1 that grows faster at the boundaries of sections I–II and II–III corresponding to the centers of MRs); compositional disorder in the А sublattice of an SS when 0.10 ≤ х ≤ 0.30 and the clusterization of its structure at the MR threshold (dielectric dispersion, which is maximal in the range of х, where the cluster splits with the matrix); redox processes of Ti4+ ↔ Ti3+, which lead to Maxwell–Wagner effects (high-temperature anomalies in ε/ε0); and the change in the relation of Fe–Pe PTs (the weakening of hysteresis phenomena). CONCLUSIONS Our results should be considered in developing functional materials and devices based on BST ceramics.
ACKNOWLEDGMENTS This work was supported by the RF Ministry of Education and Science as part of a State Task, project no. 1927, topic nos. 213.01-11/2014-21, 213.012014/012-VG, 3.1246.2014/K, SP-3197.2016.3, and МK-3232-2015-2. It was performed on equipment at the Electromagnetic, Electromechanical, and Thermal Characteristics of Solid Bodies shared resource center, Research Institute of Physics, Southern Federal University. REFERENCES 1. Mukhortov, V.M. and Yuzyuk, Yu.I., Geterostruktury na osnove nanorazmernykh segnetoelektricheskikh plenok: poluchenie, svoistva i primenenie (Heterostructures Based on Nanosized Ferroelectric Films: Fabrication, Properties, and Applications), Rostov-on-Don: Yuzhn. Nauchn. Tsentr Ross. Akad. Nauk, 2008. 2. Lemanov, V.V., Smirnova, E.P., and Tarakanov, E.A., Fiz. Tverd. Tela, 1995, vol. 37, no. 8, p. 2476. 3. Lemanov, V.V., Smirnova, E.P., Syrnikov, P.P., and Tarakanov, E.A., Phys. Rev. B, 1996, vol. 54, no. 5, p. 3151.
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Translated by A. Muravev
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No. 11
2016
