High-pressure scandium oxide and its place in the molar volume relationships of dense structures ofM2X3andABX3type

Abstract

Sc2O3 has been transformed at 1000°C and 130 kb from the relatively open C-rare-earth structure type to the monoclinic B-rare-earth form with unit cell dimensions α = 13.173 A, b = 3.194 A, c = 7.976 A, and β = 100.40°. The calculated density, 4.16 g/cm3, represents an increase of 8.3% over the density of C-type Sc2O3 and reflects an increase in coordination from six to seven fold for two-thirds of the scandium atoms. Under the same conditions In2O3 was transformed to a corundum form, but a corundum form of Sc2O3 could not be obtained at any intermediate pressure. Graphs of average metal-oxygen bond length versus molar volume, using these high-pressure phases as key examples and six-coordinate bond lengths as reference points, have enabled us to define the molar volume for a given M2X3 or ABX3 composition in any of the denser structural forms. These forms are, in order of decreasing molar volume, defect spinel; C rare earth; garnet; rare-earth manganite; corundum, which is also equivalent to ilmenite and metal-deficient nickel arsenide; B rare earth; A rare earth; and perovskite. By fitting reported shock-wave data to these graphs we conclude that very high-pressure Fe2O3 is most probably a perovskite, with possible composition Fe2+Fe4+O4, and that very high-pressure MgSiO3 is probably an ilmenite.