PSEUDOPOTENTIAL PERIODIC HARTREE-FOCK STUDY OF THE CRISTOBALITE TO STISHOVITE PHASE-TRANSITION

Silicon is tetracoordinated in most silica polymorphs except stishovite, in which it is hexacoordinated. Pseudopotential periodic Hartree-Fock calculations have been performed to study a possible reaction pathway between cristobalite and stishovite. The calculated transition pressure (6GPa) is consistent with the stishovite synthesis pressure range, whereas the activation enthalpy at 0 K is predicted to be of the order of 125 kJ per SiO2 mole under that pressure. Electron density has been plotted for different geometries intermediate between low-cristobalite and stishovite equilibrium structures. The ionicity increases and the electron density around oxygen and silicon atoms becomes more spherical as the system becomes hexacoordinated.