Ab initio perturbed ion description of the equation of state and a high pressure phase transition of Al 2O 3

Abstract

The electronic structure, equilibrium geometry, and equation of state of Al 2O 3 in the α — Al 2O 3 and Rh 2O 3(II) phases have been determined by means of the ab initio perturbed ion method augmented by a recently developed non-empirical Debye-like model for describing the vibrational thermal effects. A systematic multidimensional optimization of the structural parameters has been completed for the two phases from zero to 2200 K, and pressures up to 70 GPa for the α — Al 2O 3 phase, and 20 GPa for the high pressure phase. Our structural results are in good agreement with the experimental information available. Although the α — Al 2O 3 phase is more stable in the static ( p = T = 0) description, a pressure-induced phase transition is detected at room temperature near 4 GPa. The critical pressure p c increases very slightly with temperature, being about 5.5 GPa at 1000 K. This value is consistent with the phase transition found in Rh 2O 3 by Shannon and Prewitt [ Journal of Solid State Chemistry 2, (1970) 134] at p c 6.5 GPa. Furthermore, the α — Al 2O 3 phase shows a slight pressure-induced anisotropy, since the c a ratio decreases from 2.638, at zero pressure, to 2.474 at 70 GPa. Also, according to our calculations, the crystal maintains its symmetry and reduces moderately its compressibility in the high temperature regime. The present methodology gives detailed structural information on phase transitions of the type described in this work, as well as a meaningful description of the high pressure phase, a task rather difficult to undertake in the laboratory.