X-RAY DIFFRACTION OF RUBY (Al//2O//3: Cr**3** plus ) TO 175 GPa.

Compression data for ruby (Al//2O//3, approximately equals 0. 05% Cr** plus **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 R//1 minus R//2 luminescence under nonhydrostatic compression. The unit-cell volume measured at the highest pressure (where V/V//0 equals 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.