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Phase transitions in Al2O3. between corundum Rh2O3(II)-type and CaIrO3-type (post-perovskite-type) phases were examined at high pressure and high temperature in a laser-heated diamond-anvil cell (DAC) based on in situ X-ray diffraction measurements. The locations of corundum-Rh2O3(II) and Rh2O3(II)-CaIrO3 boundaries were precisely determined by conducting both forward and backward reaction experiments. The results demonstrate that corundum undergoes a phase transition toRh2O3(II)-type structure above 106 GPa at 1800 K with a negative Clapeyron slope of -6.5 ± 1.5 MPa/K, generally consistent with earlier experimental and theoretical works. The Rh2O3(II)-type phase further transforms into CaIrO3-type above 170 GPa at 2300 K, indicating the transition pressure much higher than earlier experimental work but in agreement with reported GGA calculations. The Clapeyron slope of this phase transition was found to be high negative (-20 ± 5 MPa/K). [ABSTRACT FROM AUTHOR]

Phase transitions in Al2O3. between corundum Rh2O3(II)-type and CaIrO3-type (post-perovskite-type) phases were examined at high pressure and high temperature in a laser-heated diamond-anvil cell (DAC) based on in situ X-ray diffraction measurements. The locations of corundum-Rh2O3(II) and Rh2O3(II)-CaIrO3 boundaries were precisely determined by conducting both forward and backward reaction experiments. The results demonstrate that corundum undergoes a phase transition toRh2O3(II)-type structure above 106 GPa at 1800 K with a negative Clapeyron slope of -6.5 ± 1.5 MPa/K, generally consistent with earlier experimental and theoretical works. The Rh2O3(II)-type phase further transforms into CaIrO3-type above 170 GPa at 2300 K, indicating the transition pressure much higher than earlier experimental work but in agreement with reported GGA calculations. The Clapeyron slope of this phase transition was found to be high negative (-20 ± 5 MPa/K). [ABSTRACT FROM AUTHOR]

We have measured Raman spectra of the postperovskite (PPv) phase in CaIrO3 up to 30 GPa to constrain the Grüneisen parameter (γ). We identified a total of 4 strong modes between 200 and 650 cm-1, which is in contrast with the Raman spectra of Mn2O3 and MgGeO3-PPv where at least nine different modes have been detected. We found no sign of a phase transition in the Raman spectra of PPv CaIrO3, which supports the stability of the PPv phase up to 30 GPa and room temperature in CaIrO3. The spectroscopic Grüneisen parameter, γsp,0 = 1.66~1.72, constrained from our Raman data, is in excellent agreement with the thermodynamic Grüneisen parameter, γth,0 = 1.75 ± 0.05, calculated from recent XRD measurements (Martin et al. 2007) on CaIrO3-PPv synthesized at high pressure and temperature similar to our starting material. Our result suggests that γsp constrained by Raman measurements provides a reasonable estimate on the γth of the PPv phase in CaIrO3. [ABSTRACT FROM AUTHOR]

Single crystals of CaIrO3 were prepared via flux growth method. Crystal structure parameters, including the anisotropic displacement parameters, are determined based on a single-crystal X-ray diffraction experiment. The unit-cell dimensions are a = 3.147(2), b = 9.866(6), and c = 7.302(5) Å. The structure is a three-dimensional dense structure with small vacant spaces. The CaIrO3 structure can be described as a pseudo-one-dimensional oxide and is compared with Ca4IrO6 structure. The IrO6 octahedra are significantly distorted, in contrast to other octahedral Ir4+ compounds. The O-O distances for faces and edges shared between polyhedra are shorter than other non-shared edge distances. These effects are explained by Pauling's rules and occur to decrease the repulsion between the cations. Thermal vibrations of Ca and Ir atoms are significantly anisotropic. Thermal vibrations of Ca and Ir atoms are restricted in orientation toward the shared face, shared edges, and shortest cation-cation directions. The single-crystal experiment shows that CaIrO3 crystals grow fastest along the a axis and that they assume a prism or needle shape. Strongly preferred orientation of such prism shaped CaIrO3-type post perovskite MgSiO3 crystals may develop under the shear flow in the Earth's mantle. [ABSTRACT FROM AUTHOR]

Single crystals of CaIrO3 were prepared via flux growth method. Crystal structure parameters, including the anisotropic displacement parameters, are determined based on a single-crystal X-ray diffraction experiment. The unit-cell dimensions are a = 3.147(2), b = 9.866(6), and c = 7.302(5) Å. The structure is a three-dimensional dense structure with small vacant spaces. The CaIrO3 structure can be described as a pseudo-one-dimensional oxide and is compared with Ca4IrO6 structure. The IrO6 octahedra are significantly distorted, in contrast to other octahedral Ir4+ compounds. The O-O distances for faces and edges shared between polyhedra are shorter than other non-shared edge distances. These effects are explained by Pauling's rules and occur to decrease the repulsion between the cations. Thermal vibrations of Ca and Ir atoms are significantly anisotropic. Thermal vibrations of Ca and Ir atoms are restricted in orientation toward the shared face, shared edges, and shortest cation-cation directions. The single-crystal experiment shows that CaIrO3 crystals grow fastest along the a axis and that they assume a prism or needle shape. Strongly preferred orientation of such prism shaped CaIrO3-type post perovskite MgSiO3 crystals may develop under the shear flow in the Earth's mantle. [ABSTRACT FROM AUTHOR]

The structure of CaIrO3 (Cmcm) has been refined at high pressure and at low temperature using time-of-flight neutron powder diffraction data. Evidence supporting deviation from space group Cmcm to Cmc21 is inconclusive. As CaIrO3 (Cmcm) unit-cell volume changes, refinements indicate deformation of cation-centered coordination polyhedra, rather than tilting. Structure models demonstrate Ca2+-centered polyhedra are an order of magnitude more compressible than Ir4+-centered octahedra. Bond valence sums show significant chemical strain (over-bonding) of calcium and oxygen at ambient conditions. Implications for structure change in MgSiO3 post-perovskite are discussed and a method for predicting the Clapeyron slope between perovskite and post-perovskite phases is proposed based on extrapolation of the volume-ratio between cation-centered polyhedra. [ABSTRACT FROM AUTHOR]

The effect of the anisotropic connectivity within the layers of the post-perovskite, CaIrO3, structure are examined using a Rigid Unit Mode and group theory approach, and possible commensurate tilt structures are given. It is shown that the anisotropic bonding of the sheet may give rise to preferred directions of buckling for such structures and anisotropy in the speed of sound. [ABSTRACT FROM AUTHOR]

Analysis of pressure-temperature dependent monochromatic X-ray powder diffraction data yield the bulk modulus [KT = 180.2(28) GPa] and thermal expansion coefficients [α0 = 2.841(34) x 105 K-1; α1 = 3.37(48) x 10-9 K-2] of CaIrO3, the structure model for post-perovskite MgSiO3. CaIrO3 is orthorhombic (Cmcm, space group 63, Z = 4) with best-fit unit-cell parameters, a = 3.14147(5) Å, b = 9.87515(19), c = 7.29711(11), and V = 226.3754(78) ų at 1 bar and 300 K. The c-axis of CaIrO3 has a small compressibility and a large thermal expansion when compared to the other principal axes. Rietveld structure refinement reveals changes in CaIrO3 as a function of temperature in terms of IrO6 octahedra distortion. Dissociation of CaIrO3 at high temperature has possible implications for the post-perovskite MgSiO3 structure, Earth's lower mantle, and D" layer. [ABSTRACT FROM AUTHOR]