High-pressure transformation of Al2O3

X-ray diffraction measurements indicate that ruby ([Cr.sup.3+] doped [Alpha]-[Al.sub.2O.sub.3]) transforms to the [Rh.sub.2O.sub.3] (II) Structure when heated to temperatures exceeding ~1000 kelvin at pressures above ~100 gigapascals, in agreement with predictions from ab initio quantum mechanical calculations. The high-pressure phase did not quench upon decompression to ambient pressure, and the occurrence of this phase transformation may affect interpretations of static (diamond-anvil cell) and dynamic (shock-wave) experiments at ultra-high pressures.