Thermal expansion of CaIrO3 determined by X-ray powder diffraction

The thermal expansion of CaIrO3, the archetype phase of the “post-perovskite” polymorph of MgSiO3, has been determined by X-ray powder diffraction from 113 to 1173 K. For temperatures above 298 K, the volumetric coefficient of thermal expansion, α(T), can be represented by α(T) = a0 + a1(T), with a0 = 2.42(3) × 10−5 K−1 and a1 = 0.73(4) × 10−8 K−2. The data for the full range of temperature investigated can be fitted well by a second-order Grüneisen approximation to the zero-pressure volumetric equation of state, with the internal energy calculated via a Debye model, and parameters: θD = 703(11) K, V0 = 225.666(6) Å3, K′0=4.8(4)K′0=4.8(4) and (V0K0/γ′) = 2.77(2) × 10−17 J, where θD is the Debye temperature, V0 the unit-cell volume at T = 0 K, K′0K′0 the first derivative with respect to pressure of the incompressibility, K0, and γ′ is a Grüneisen parameter. The results of similar procedures, applied to the individual lattice parameters, suggest that the degree of elastic anisotropy shown by CaIrO3 at ambient pressure may be much greater than that calculated for the high-pressure post-perovskite polymorph of MgSiO3.