Phase equilibria, crystal structures, and dielectric anomaly in the BaZrO3–CaZrO3 system

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Abstract

Phase equilibria in the (1−x)BaZrO3xCaZrO3 system were analyzed using a combination of X-ray and neutron powder diffraction, and transmission electron microscopy. The proposed phase diagram features two extended two-phase fields containing mixtures of a Ba-rich cubic phase and a tetragonal, or orthorhombic Ca-rich phase, all having perovskite-related structures. The symmetry differences in the Ca-rich phases are caused by different tilting patterns of the [ZrO6] octahedra. In specimens quenched from 1650°C, CaZrO3 dissolves only a few percent of Ba, whereas the solubility of Ca in BaZrO3 is approximately 30at%. The BaZrO3–CaZrO3 system features at least two tilting phase transitions, Pm3mI4/mcm and I4/mcmPbnm. Rietveld refinements of the Ba0.8Ca0.2ZrO3 structure using variable-temperature neutron powder diffraction data confirmed that the Pm3mI4/mcm transition corresponds to a rotation of octahedra about one of the cubic axes; successive octahedra along this axis rotate in opposite directions. In situ variable-temperature electron diffraction studies indicated that the transition temperature increases with increasing Ca-substitution on the A-sites, from approximately −120°C at 5at% Ca to 225°C at 20at% Ca. Dielectric measurements revealed that the permittivity increases monotonically from 36 for BaZrO3 to 53 for Ba0.9Ca0.1ZrO3, and then decreases to 50 for Ba0.8Ca0.2ZrO3. This later specimen was the Ca-richest composition for which pellets could be quenched from the single-phase cubic field with presently available equipment. Strongly non-monotonic behavior was also observed for the temperature coefficient of resonant frequency; however, in this case, the maximum occurred at a lower Ca concentration, 0.05⩽x⩽0.1. The non-linear behavior of the dielectric properties was attributed to two competing structural effects: a positive effect associated with substitution of relatively small Ca cations on the A-sites, resulting in stretched Ca–O bonds, and a negative effect, related to the distortion of the A-site environment (bond strain relaxation) upon octahedral tilting.

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 (tanδ). 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 20mol% 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 Pm3m(a=ac) perovskite structure [5], while both CaZrO3 [6] and SrZrO3 [7] exhibit orthorhombic Pbnm(√2ac×√2ac×2ac) symmetry determined by rotation (bbc+ 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)BaZrO3xCaZrO3 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)BaZrO3xCaZrO3 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)BaZrO3xCaZrO3 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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