Search

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]

Abstract: The thermal diffusivity (κ) of perovskite and post-perovskite CaIrO3 has been measured, at elevated pressure and temperatures up to 600°C, using the X-radiographic Ångström method. At high temperatures we find that the thermal diffusivity of post-perovskite is slightly below twice that of isochemical perovskite over the temperature range investigated. Assuming a similar effect occurs in MgSiO3 post-perovskite, the effect of the contrasting thermal transport properties between perovskite and post-perovskite on mantle dynamics has been investigated using simple two-dimensional convection models. These show a reduction in extent and increase in depth of post-perovskite lenses, as well as increased core–mantle-boundary heat-flux, broader upwellings and more vigorous downwellings when compared to the reference, constant a, case. [Copyright &y& Elsevier]

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]