A new high-pressure polymorph of Ti_2O_3: implication for high-pressure phase transition in sesquioxides

摘要

The post-corundum phase transition has been investigated in Ti_2O_3 on the basis of synchrotron X-ray diffraction in a diamond anvil cell and transmission electron microscopy. The new polymorph of Ti_2O_3 was found at about 19 GPa and 1850 K, and this phase was stable even at about 40 GPa. A new polymorph of Ti_2O_3 can be indexed on a Pnma orthorhombic cell, and the unit-cell parameters are a = 7.6965 (19)A, b = 2.8009 (9)A, c = 7.9300 (23)A, V = 170.95 (15)A~3 at 19 GPa, and a = 7.8240 (2)A, b = 2.8502 (1)A, c = 8.1209 (3)A, V = 181.10 (1)A~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 Ti_2O_3 can be identified as Th_2S_3-type rather than U_2S_3-type structure. The transition from corundum-type to Th_2S_3-type structure accompanies the drastic change of the form of polyhedron: from TiO_6 octahedron in the corundum-type to TiO_7 polyhedron in the Th_2S_3-type structures.