PERIODIC HARTREE-FOCK INVESTIGATION OF THE STISHOVITE CACL2-LIKE PHASE-TRANSITION OF SILICA

Periodic Hartree-Fock calculations have been performed in order to inv estigate the stishovite-CaCl2-like structure phase transition observed for silica above 100 GPa. The mechanism involves the B1g (GAMMA3+) mo de which corresponds to the rotation of the SiO6 octahedra around thei r C2 axes. The B1g frequency presents negative pressure shifts. It is shown that the potential energy surface with respect to the relevant n ormal coordinate appears to be a symmetric single well at low pressure s whereas at pressures higher than 80 GPa it becomes a symmetric doubl e well. This explains why the CaCl2-like structure is unquenchable. Mo reover we predict for this silica modification an order-disorder phase transition at high temperatures.