A FIRST-PRINCIPLES HARTREE-FOCK DESCRIPTION OF MNO AT HIGH-PRESSURES

First-principles periodic Hartree-Fock calculations of MnO are reporte d which suggest that the phase transition recently observed under shoc k compression at similar to 90 GPa is not a simple B1-to-B2 transition of the type observed in CaO and SrO. The insulating B1 phase, which i s rhombohedrally distorted as a result of spin-lattice interaction, is predicted to persist up to the phase transition pressure. Calculated Hugoniot curves based on a ferromagnetic Fm3m structure, with and with out the inclusion of a posteriori correlation corrections, and a rhomb ohedrally distorted AF(II) structure are in close agreement with earli er static compression data. Beyond this pressure, calculations suggest that the phase is either an insulating distorted B2 phase or a metall ic B2 phase.