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

Phase equilibria in the (1 - x)BaZrO3-xCaZrO(3) 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 1650degreesC, CaZrO3 dissolves only a few percent of Ba, whereas the solubility of Ca in BaZrO3 is approximately 30 at%. The BaZrO3-CaZrO3 system features at least two tilting phase transitions, Pm3m --> 14/mcm and 14/mcm --> Pbnm. Rietveld refinements of the Ba0.8Ca0.2ZrO3 structure using variable-temperature neutron powder diffraction data confirmed that the Pm3m --> 14/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 - 120degreesC at 5 at% Ca to 225degreesC at 20 at% 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 less than or equal to x less than or equal to 0.l. 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. (C) 2003 Elsevier Inc. All rights reserved.