IONIC SOLIDS AT ELEVATED-TEMPERATURES AND HIGH-PRESSURES - MGF2

A combination of periodic Hartree-Fock theory, quasiharmonic lattice d ynamics, and molecular dynamics is used to study the behavior of MgF2 at elevated temperatures and/or high pressures, Particular attention i s paid to the pressure-induced transition from the rutile to the fluor ite structure in view of earlier theoretical estimates of the transiti on pressure, which differ widely. It is shown that previously reported potentials obtained by fitting to empirical data fail to reproduce th ermodynamic properties. To rectify this, a new set of consistent two-b ody potentials has been derived from nb initio periodic Hartree-Fock c alculations. Lattice dynamics calculations in the quasiharmonic approx imation based on these potentials has been used to study the two phase s of MgF2 at high T and P. The resulting transition pressure and that obtained directly from Hartree-Fock calculations in the static limit a re both less than or equal to 30 GPa, which is close to the experiment al value but appreciably lower than a previous molecular dynamics valu e of over 130 GPa. The variation of quantities such as (partial deriva tive P/dT)(V) which play a central role in the formulation of approxim ate equations of state is also considered. (C) 1997 American Institute of Physics.