A new high-pressure polymorph of Ti2O3: implication for high-pressure phase transition in sesquioxides

The post-corundum phase transition has been investigated in Ti2O3 on the basis of synchrotron X-ray diffraction in a diamond anvil cell and transmission electron microscopy. The new polymorph of Ti2O3 was found at about 19 GPa and 1850 K, and this phase was stable even at about 40 GPa. A new polymorph of Ti2O3 can be indexed on a Pnma orthorhombic cell, and the unit-cell parameters are a = 7.6965 (19) angstrom, b = 2.8009 (9) angstrom, c = 7.9300 (23) angstrom, V = 170.95 (15) angstrom(3) at 19 GPa, and a = 7.8240 (2) angstrom, b = 2.8502 (1) angstrom, c = 8.1209 (3) angstrom, V = 181.10 (1) angstrom(3) at ambient conditions. The Birch-Murnaghan equation of state yields K-0 = 206 (3) GPa and K'(0) = 4 (fixed) for corundum phase, and K-0 = 296 (4) GPa and K'(0) = 4 (fixed) for the post-corundum phase. The molar volume decreases by 12% across the phase transition at around 20 GPa. The structural identification was carried out on a recovered sample by the Rietveld method, and a new polymorph of Ti2O3 can be identified as Th2S3-type rather than U2S3-type structure. The transition from corundum-type to Th2S3-type structure accompanies the drastic change of the form of polyhedron: from TiO6 octahedron in the corundum-type to TiO7 polyhedron in the Th2S3-type structures.