Phase Equilibria, Crystal Structures, and Dielectric Anomaly in the BaZrO3-CaZrO3 System

Published

November 1, 2003

Author(s)

Igor Levin, T G. Amos, S M. Bell, L Farber, Robert S. Roth, Terrell A. Vanderah, B H. Toby

Abstract

Phase equilibria in the (1-x)BaZrO₃-xCaZrO₃ 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 atom fraction solubility of Ca in BaZrO₃ is approximately 30%. The BaZrO₃-CaZrO₃ system features at least two tilting phase transitions, Pm3m--> I4/mcm and I4/mcm-->Pbnm. Rietveld refinements of the Ba_0.8Ca_0.2ZrO₃ structure using variable-temperature neutron powder diffraction data confirmed that the Pm3m--->I4/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 indicates formation the transition temperature increases with increasing Ca-substitution on the A-sites, from approximately 120 C at 5% atom fraction Ca to 225 C at 20 % Ca. Dielectric measurements revealed that the permittivity increases monotonically from 36 for BaZrO₃ to 53 for Ba_0.9Ca_0.1ZrO₃, and then decreases to 50 for Ba_0.8Ca_0.2ZrO3. This latter specimen was the Ca-richest composition for which pellets could be quenched from the single-phase cubic field with presently available equipment.

Citation

Journal of Solid State Chemistry

Volume

175

Issue

No. 2

Pub Type

Journals

Keywords

Bond strain, crystal structure, dielectric properties, octahedral tilting, perovskites, zirconates

Citation

Levin, I. , Amos, T. , Bell, S. , Farber, L. , Roth, R. , Vanderah, T. and Toby, B. (2003), Phase Equilibria, Crystal Structures, and Dielectric Anomaly in the BaZrO₃-CaZrO₃ System, Journal of Solid State Chemistry (Accessed September 10, 2024)