First-principles calculations of the ferroelastic transition between rutile-type and CaCl2-type SiO2 at high pressures

The tetragonal to orthorhombic ferroelastic phase transition between rutile- and CaCl2-type SiO2 at high pressures is studied using first-principles calculations and the Landau free-energy expansion. The phase transition is systematically investigated in terms of characteristic phonon modes with B-1g and A(g) symmetries, shear moduli, transverse-acoustic mode, rotation angle of the SiO6 octahedra, spontaneous symmetry-breaking and volume strains, and enthalpy. The results show that these physical behaviors at the transition are well described using the Landau free-energy expansion parametrized by the first-principles calculations.