The equation of state of CaSiO3 perovskite to 108 GPa at 300 K

Pressure-volume measurements have been performed for CaSiO3 perovskite to 1 08 GPa at 300 K using NaCl and argon pressure-transmitting media, and energ y dispersive X-ray diffraction (EDXD) in a diamond-anvil cell (DAC). By det ermining a parameter that is the product of the elastic anisotropy, S, and the uniaxial stress component, t, for each data point, we define the stress condition of the sample, For different points at the same pressure in a te mperature-quenched sample, the St value can differ by as much as a factor o f 5, indicating heterogeneity in the stress condition. This may be responsi ble for the large scatter of earlier P-V measurements in the DAC which in g eneral used a large diameter X-ray beam. Also, the St value provides insigh t into the elastic anisotropy, S, of CaSiO3 perovskite and platinum. The si gn of S (positive) for CaSiO3 perovskite agrees with first principles calcu lations but the magnitude may be inconsistent. A new compression curve at 3 00 K was obtained for CaSiO3 perovskite by using those data points which re present the most nearly hydrostatic conditions. It is observed that the dat a points with high St values yield larger volumes than the points with smal l St values at a given pressure. By selecting the data points having low St values (St less than or equal to 0.005), combining with lower pressure lar ge volume press (LVP) measurements and fitting to third order Birch-Murnagh an equation of state (EOS), we find that CaSiO3 perovskite is more compress ible (V-0 = 45.58 +/- 0.05 Angstrom(3) K-T0 = 236 +/- 4 GPa, and K'(T0) = 3 .9 +/- 0.2 GPa) than suggested by previous studies. The density and bulk mo dulus of CaSiO3 perovskite at lower mantle pressures and 300 K are 1-3% gre ater and 5-15% smaller, respectively, than found in previous studies. This study demonstrates that defining the stress state of the sample is crucial to obtain an accurate 300 K compression curve for unquenchable high-pressur e phases. (C) 2000 Elsevier Science B.V. All rights reserved.