Phase equilibria, crystal structures, and dielectric anomaly in the BaZrO3–CaZrO3 system
Introduction
Complex oxides with perovskite-like structures are attractive candidates for use in wireless communication applications, which require a combination of high permittivity (ε), near-zero temperature coefficient of the resonant frequency (τf), and low dielectric loss tangent . Many recent studies of such oxide systems have focussed on the phase equilibria and dielectric properties of perovskite-related solid solutions having CaTiO3, SrTiO3 or BaTiO3—all of which exhibit large permittivities—as one of the end-compounds [1], [2], [3]. In contrast, AZrO3-based perovskites, which exhibit much lower permittivities than their titanate analogs, have received less attention. An interesting dielectric anomaly has been reported for the CaZrO3(CZ)–SrZrO3(SZ)–BaZrO3(BZ) system: Yamaguchi et al. [4] observed that both ε and τf in the BZ–CZ system exhibit maxima at CaZrO3; in contrast, these properties change monotonically between the end-compounds in the other two binary systems. Despite this anomalous difference in dielectric behavior between the BZ–CZ and both CZ–SZ and BZ–SZ ceramics, the detailed phase equilibria and structural behavior in the three systems have not been clarified.
The room-temperature crystal structures of all three end-compounds have been reported in the literature: BaZrO3 crystallizes with an ideal cubic perovskite structure [5], while both CaZrO3 [6] and SrZrO3 [7] exhibit orthorhombic symmetry determined by rotation (b−b−c+ type according to Glaezer's notation [8]) of the oxygen octahedra. Recently, Kennedy et al. [9] conducted a detailed analysis of tilting phase transitions in SZ–BZ perovskite solid solutions; however, no such studies have yet been reported for either the BZ–CZ or CZ–SZ systems. Yamaguchi et al. [4] presented some results on phase assemblages in the ternary CZ–BZ–SZ system. In particular, they reported the existence of cubic and orthorhombic perovskite solid solutions in the (Ba, Sr)-rich and (Sr, Ca)-rich regions, respectively, as well as their mixture in the (Ba, Ca)-rich part of the diagram; however, the results presented in that study were not sufficient to construct a phase diagram. In the present work, X-ray and neutron powder diffraction combined with transmission electron microscopy were applied to analyze phase equilibria and structural details in the BZ–CZ system, especially in regions associated with the dielectric anomaly. Dielectric properties were measured for selected compositions and correlated with the observed structural behavior.
Section snippets
Experiment
Polycrystalline samples in the (1−x)BaZrO3– xCaZrO3 system were prepared by solid state reaction of CaCO3 (Alfa-Aesar,1 99.99%), BaCO3 (Prochem Inc., 99.999%), and ZrO2 (TAM, low Hf ) powders. Stoichiometric amounts were first ground with acetone using an agate mortar and pestle. Mixtures were pressed into pellets and placed on beds of
Ca solubility limit in BaZrO3
X-ray powder diffraction analyses of (1−x)BaZrO3–xCaZrO3 specimens with x=0.05, 0.1, 0.12, 0.2, 0.3, 0.4 and 0.5 were used to establish the T(x) line separating a high-temperature Ba-rich cubic phase from a two-phase region containing orthorhombic CaZrO3, as shown in Fig. 1. X-ray diffraction patterns for both the x=0.5 and 0.4 specimens quenched from temperatures up to 1650°C could be indexed according to a mixture of orthorhombic CaZrO3 and cubic BaZrO3-like phases (Fig. 2); the results agree
Conclusions
Phase equilibria in the (1−x)BaZrO3–xCaZrO3 system were investigated using a combination of transmission electron microscopy with X-ray and neutron powder diffraction. The equilibrium phase diagram for this system features extended two-phase fields representing mixtures of a cubic Ba-rich phase and a tetragonal, or orthorhombic (in order of decreasing temperature), Ca-rich phase; all phases crystallize with perovskite-related structures. As expected, the solubility of Ba in CaZrO3 is limited to
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2023, Ceramics InternationalCitation Excerpt :Detail of experimental method is described in our previous report [16]. Additional data of AIIBIVO3 perovskite oxides (110 sets of 18 compositions) were collected from the experimental information of literature data (CaTiO3 [21], BaZrO3 [22], CaZrO3 [22], Ba0.8Ca0.2ZrO3 [22], SrTi0.5Zr0.5O3 [23], SrTi0.75Zr0.25O3 [23], SrHfO3 [24], Sr1-xBaxHfO3 [25], SrRuO3 [26]). A total of 1017 sets of 58 compositions (including chemical compositions, measured temperature, measured atmosphere) were systematically collected, and the statistics of space groups is shown in Fig. 1.