High-pressure phase transitions of CaRhO3 perovskite

High-pressure phase transitions of CaRhO3 perovskite were examined at pressures of 6-27 GPa and temperatures of 1,000-1,930A degrees C, using a multi-anvil apparatus. The results indicate that CaRhO3 perovskite successively transforms to two new high-pressure phases with increasing pressure. Rietveld analysis of powder X-ray diffraction data indicated that, in the two new phases, the phase stable at higher pressure possesses the CaIrO3-type post-perovskite structure (space group Cmcm) with lattice parameters: a = 3.1013(1) , b = 9.8555(2) , c = 7.2643(1) , V (m) = 33.43(1) cm(3)/mol. The Rietveld analysis also indicated that CaRhO3 perovskite has the GdFeO3-type structure (space group Pnma) with lattice parameters: a = 5.5631(1) , b = 7.6308(1) , c = 5.3267(1) , V (m) = 34.04(1) cm(3)/mol. The third phase stable in the intermediate P, T conditions between perovskite and post-perovskite has monoclinic symmetry with the cell parameters: a = 12.490(3) , b = 3.1233(3) , c = 8.8630(7) , beta = 103.96(1)A degrees, V (m) = 33.66(1) cm(3)/mol (Z = 6). Molar volume changes from perovskite to the intermediate phase and from the intermediate phase to post-perovskite are -1.1 and -0.7%, respectively. The equilibrium phase relations determined indicate that the boundary slopes are large positive values: 29 +/- A 2 MPa/K for the perovskite-intermediate phase transition and 62 +/- A 6 MPa/K for the intermediate phase-post-perovskite transition. The structural features of the CaRhO3 intermediate phase suggest that the phase has edge-sharing RhO6 octahedra and may have an intermediate structure between perovskite and post-perovskite.