Geometry optimization and ground-state properties of complex ceramic oxides

The crystal structures of the several complex oxides including MgAl2O4, alp ha -Al2O3, theta -Al2O3, r-Al2O3, Y2O3, Y3Al5O12, and YAIO(3) are fully opt imized by means of first-principles total energy calculations with relaxati on of all internal parameters. This is achieved by the implementation of a simple scheme based on the finite difference method for the energy gradient . In most cases, the predicted lattice constants are within 0.5% of the exp erimental values, The bulk moduli and the pressure coefficients for these c rystals are calculated from fitting the energy vs volume data to the equati on of state. Because all of the internal parameters of the crystal are opti mized at different volumes, the calculated ground-state properties are foun d to be in excellent agreement with measured data. For the polymorphs of th e Al2O3 system, comparison to other existing calculations are made and disc ussed.