Polymorphous modifications of silica in the earth's lower mantle

A calculation method in which a crystalline body is considered as a collection of anharmonic oscillators is used to construct the phase diagram of SiO2 and to evaluate the existence limits of high-density modifications of silica with alpha-PbO2 and fluorite structure in the lower mantle. It is shown that SiO2 phases with alpha-PbO2 and CaF2 structure are unstable in lower-mantle conditions (i.e., at temperatures exceeding 1000 C and at pressures below 300 GPa).