Thermal expansion of SrZrO3 and BaZrO3 perovskites

High-temperature X-ray diffraction studies of SrZrO₃ and BaZrO₃ perovskites have been carried out to 1200° C. The diffraction patterns are analyzed with Rietveld method so as to refine the unit cell dimensions. The volumetric thermal expansion coefficient are observed to be 2.98*10^-5K^-1 for orthorhombic Pbnm phase, 3.24*10^-5K^-1 for orthorhombic Cmcm phase, 3.75*10^-5K^-1 for tetragonal I4/mcm phase of SrZrO₃ perovskite, and 2.06*10^-5K^-1 for cubic Pm3m phase of BaZrO₃ perovskite, respectively. The linear thermal expansion coefficients of SrZrO₃ perovskite show considerable anisotropy of α _a >α _c >α _b for orthorhombic Pbnm phase, which reflect the decrease of distortion of the perovskite. It is demonstrated that thermal expansion of the centrosymmetrically distorted ABX₃ perovskite can be empirically expressed as a combination of the changes of [B-X] bond length and tilting angle of BX₆ octahedral framework. The octahedral tilting is considered to be the primary order parameter for the ferroelastic type of structural phase transitions in perovskite. Thermodynamically, the tilting induced volume change denotes the “excess volume” and the corresponding thermal expansion represents the “excess thermal expansion” for the lower symmetry phase with respect to its prototype of the cubic perovskite.