Equation of state and high‐pressure phase transitions of stishovite (SiO2): Ab initio (periodic Hartree‐Fock) results

The possibility that free SiO2 may exist at the core mantle boundary motivates a first‐principles investigation of the equation of state and possible phase transitions of stishovite using periodic Hartree‐Fock theory. Using a moderately extended basis set, the calculated equation of state is shown to be in close agreement with experimental results taken under hydrostatic conditions. When fit to a third‐order Birch‐Murnaghan equation, the Hartree‐Fock total energies give V0 = 46.1 A3, K0 = 328 GPa, and K′ = 4.0. Moreover, the calculated equation of state is very close to that recently obtained using the linearized augmented plane wave method. The bonding in stishovite is unchanged by compression to 80% of its zero‐pressure volume (corresponding to a pressure near 110 GPa). The possibility that stishovite might transform to a denser phase at high pressure is investigated by calculating the free energy of SiO2 in the modified‐fluorite and α‐PbO2 structures. The enthalpy of SiO2 with the fluorite and modified‐fluorite structure is too high to allow those structures to be adopted. The 0 K enthalpy of silica in the α‐PbO2 structure is within 10 kJ/mol of that of stishovite, and a transition to such a structure cannot be ruled out in the temperature regime of the lower mantle. However, the density of α‐PbO2 structure SiO2 is only 2% greater than that of stishovite.