The pressure dependence of the dielectric and dynamical properties of ${\mathrm{SiO}}_2$ stishovite and its pressure-induced ferroelastic phase transition to the ${\mathrm{CaCl}}_2$ 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 $B_{1g}$ 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 ${\mathrm{CaCl}}_2$ 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 ${\mathrm{CaCl}}_2$ structure.

• Received 11 November 1996

DOI:https://doi.org/10.1103/PhysRevB.56.7321