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.

Keywords:

• high pressure
• phase transition
• sesquioxides
• multi-anvil synthesis
• Cr₂O₃
• Ga₂O₃
• corundum structure
• pressure calibrant

Acknowledgements

The authors are grateful to K. Glazyrin for the assistance in the single-crystal XRD study on Cr₂O₃. S.V.O. thanks the Alexander von Humboldt Foundation for the financial support.