SiO2 stishovite under high pressure: Dielectric and dynamical properties and the ferroelastic phase transition

Abstract

The pressure dependence of the dielectric and dynamical properties of SiO2 stishovite and its pressure-induced ferroelastic phase transition to the CaCl2 structure are investigated using density-functional theory. The pressure dependence of the dielectric permittivity tensors and the Born effective charges shows that the local atomic environments in stishovite, more compressible along the a axis than along the c axis, become less anisotropic as pressure increases. The phonon frequencies at the Γ point increase with increasing pressure, except the B1g mode. The phonon band structure at 73 GPa, the study of elastic constants as a function of pressure, and the structural relaxation at various pressure provide evidence for spontaneous elastic deformation, i.e., a ferroelastic phase transition at 64 GPa of stishovite in the tetragonal rutile structure to the orthorhombic CaCl2 structure. The on-site and interatomic force constants are found to be consistent with the predicted structural phase transition. The dielectric properties and the phonon frequencies at the Γ point are also computed in the CaCl2 structure.