Equations of state of CaIrO3 perovskite and post-perovskite phases

Unit-cell lattice parameters have been measured to similar to 8 GPa in a diamond anvil cell for two single crystals of CalrO(3): one with the perovskite (Pbnm) and the other with the post-perovskite (Cmcm) structure. The CalrO(3) post-perovskite structure is more compressible than the perovskite. A third-order Birch Murnaghan equation of state has been used to fit the measured P- V data with the following refined parameters: V-0 = 229.463(8) angstrom(3), K-0 = 198(3) GPa, K' = 1.2(8); and V-0 = 226.38(1) angstrom(3) K-0 = 181(3) GPa, K' = 2.3(8) for CalrO(3) perovskite and post-perovskite, respectively. The compressibility of the unit-cell axes of the perovskite structure is highly aniscitropic with beta(a) >> beta(c) >> beta(b) In contrast, the b axis is the most compressible in the post-perovskite structure, whereas the a and c axes have similar compressibilities (with c slightly less compressible than a) and are much stiffer. A comparison between the compressibility of CalrO(3) perovskite and post-perovskite with the isostructural MgSiO3 phases, reveals a similar general behavior, although in detail CalrO(3) perovskite is more and the postperovskite less anisotropic than the corresponding MgSiO3 compounds.