High-pressure high-temperature synthesis of Cr2O3 and Ga2O3

Abstract

We report the results of high-pressure high-temperature (HP-HT) synthesis of two sesquioxides, Cr2O3 at 22 GPa and 1950°C and Ga2O3 at 21 GPa and 1600°C, and their X-ray diffraction and Raman studies. After HP-HT synthesis, both oxides presented assemblies of single crystallites that adopted a corundum structure. For HP-HT-synthesized single crystal of Cr2O3, we determined the lattice parameters and the atomic positions in the corundum structure as follows: a=4.9530 Å, c=13.5884 Å, Cr (12 c): 0, 0, 0.3475; O (18 c): 0.3058(5), 0, 0.25. We explained the ‘evidence’ for the existence of a ‘distorted’ corundum lattice above 13–15 GPa by resonant effects in Raman spectra. For Ga2O3, it was demonstrated that HP-HT synthesis can fully turn a sample from a monoclinic C2/m phase (β-Ga2O3) to a corundum phase (α-Ga2O3), although at ambient temperature this sluggish transition is usually completed at ∼35–40 GPa. It was found that α-Ga2O3 exhibits a very strong luminescence doublet that resembles the one in ruby, and hence could have potential for pressure calibration.