Accurate x-ray diffraction experiments for hydrostatic compression of corundum α-Al2O3 and transition-metal sesquioxides with the corundum structure, V2O3, Cr2O3, and α-Fe2O3, were performed using a cubic-anvil type of high-pressure apparatus at room temperature up to about 120 kbar. An isotropic compression of the unit cell was found in α-Al2O3 and Cr2O3. In V2O3 and α-Fe2O3, anisotropic behaviors of compression were observed: The ratio c/a in the hexagonal cell increases in V2O3 and decreases in α-Fe2O3 with increasing pressure. The isothermal bulk modulus KT=1.75±0.03 Mbar of V2O3 is unusually small among the studied compounds with the corundum structure. Values of 2.39±0.04 Mbar and 2.31±0.05 Mbar were calculated for KT of α-Al2O3 and Cr2O3, and a value of 2.31±0.10 Mbar for KT of α-Fe2O3 in the pressure range below 30 kbar.

Static-compression experiments to 59 GPa (590 kbar), employing X ray diffraction through a diamond cell, demonstrate that strontium oxide (SrO) transforms from its initial B 1 (NaCl-type) to the B2 (CsCl-type) structure at 36 (-+4) GPa on the ruby fluorescence scale with a volume change at the transition A V/V• -• -13%. These results confirm earlier predictions based upon systematics for oxides but are in lesser agreement with ab initio (modified electron gas) calculations. The new data for the B 1 phase are consistent with previous ultrasonic and static-compression measurements, which indicate that Ko' > 4. Data spanning the 25-GPa range in pressure constrain the equation of state of the B2 phase. The change in bulk sound velocity at the transition is shown to be essentially zero, which is a significant deviation from the behavior predicted by velocity-density systematics.the sample, as well as the uncertainty associated with the ruby fluorescence calibration [Mao et al., 1978]. RESULTS AND DISCUSSION

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The crystal structure of the spinel polymorph of Mg2SiO4 has been refined using a single crystal synthesized at 220 kbar and 1000°C. The u parameter of the structure with a = 8.0649(1) Å is 0.3685(6). Although the refinement indicates some cation disorder in Mg2SiO4 spinel, the results are not definitive because of the similarity in the scattering factors of Mg and Si. The average Si‐O and Mg‐O distances are 1.655(4) and 2.070(4) Å, respectively. The final R and Rw values are 0.048 and 0.040 for 41 reflections up to sin θ/λ = 1.0.

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