Compression data for ruby (Al2O3, ≈ 0.05% Cr+3) have been obtained by energy-dispersive X-ray scattering in a diamond-anvil cell to 175 GPa (1.75 Mbar) at room temperature. Ruby does not undergo any reconstructive structural transition to at least 175 GPa at room temperature, although there is comparatively large uncertainty in pressure due to the weakening and broadening of R1-R2 luminescence under nonhydrostatic compression. The unit-cell volume measured at the highest pressure (where V/V0 = 0.71) is consistent with the predictions of a Birch-Murnaghan equation of state with parameters obtained from low-pressure ultrasonic and single-crystal compression measurements (K0 ≅ 254 GPa, K'0 ≅ 4.3). Within error, the c/a ratio remains approximately constant indicating that the compression mechanism in Al2O3:Cr+3 is not strongly anistropic. Very close agreement is obtained with a first-principles calculation of the corundum equation of state at the highest pressure of the study.
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