QUANTUM-MECHANICAL CALCULATION OF THE SOLID-STATE EQUILIBRIUM MGO-AL2O3-REVERSIBLE-ARROW-MGAL2O4 (SPINEL) VERSUS PRESSURE(ALPHA)

The ground-state crystal energies of cubic MgAl2O4 (spinel) and MgO (p ericlase) and of rhombohedral alpha-Al2O3 (corundum) have been calcula ted at different volumes, relaxing the corresponding structures, by al l-electron periodic Hartree-Fock methods (CRYSTAL program). Basis sets of contracted Gaussian-type functions are employed for the 18 atomic (including d) orbitals representing each of the Mg, Al, and O atoms. M ulliken net atomic charges z(Mg)=1.86\e\ (MgO), z(Al)=2.30\e\ (alpha-A l2O3), z(Mg)=1.74\e\, and z(Al) = 2.24\e\ (spinel) are obtained. The e lastic bulk modulus, the Murnaghan equation of state p(V) at the ather mal limit, the Mg-O and Al-O bond compressibilities, and the binding e nergy have been derived for each phase (and the elastic constants C-11 and C-12 for spinel only). Comparison with existing experimental data is discussed. The enthalpy change for spinel decomposition into the s imple oxides has been computed as a function of pressure, including a correction for the electron correlation energy based on local-density- functional theory. A decomposition pressure of 11 GPa at T=0 K is pred icted, against values of 8 and 13 GPa derived from experimental thermo dynamic data and from direct compression experiments, respectively.