Structural phase transformations and new intermediate phases of MgF2 under high-pressures applied via conjugate-gradient method

Abstract

An ab initio constant pressure technique is performed in order to study the structural behavior of MgF2 under the hydrostatic pressure up to 700 GPa. Two high-pressure phases of MgF2 are successfully observed through constant pressure simulations. MgF2 undergoes a phase transformation from the rutile-type structure to the cubic-fluorite-type structure with space group Fm(3) over bar m at 280 GPa. Another phase transformation from the cubic-fluorite-type structure to the cotunnite-type structure with space group Pnma occurs at 500 GPa. The first transformation proceeds via three intermediate phases with space groups Pnnm, Immm and R(3) over bar m. We also investigate the stability of these phases from the total energy and enthalpy computations. According to these investigations, the phase transformations should occur from the rutile-type structure to the cubic-fluorite-type structure around 20 GPa and from the cubic-fluorite-type structure to the cotunnite-type structure around 45 GPa. (C) 2014 Elsevier B.V. All rights reserved.