SIO2 STISHOVITE UNDER HIGH-PRESSURE - DIELECTRIC AND DYNAMICAL PROPERTIES AND THE FERROELASTIC PHASE-TRANSITION

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 theo ry. The pressure dependence of the dielectric permittivity tensors and the Born effective charges shows that the local atomic environments i n stishovite, more compressible along the a axis than along the c axis , become less anisotropic as pressure increases. The phonon frequencie s at the Gamma point increase with increasing pressure, except the B-1 g mode. The phonon band structure at 73 GPa, the study of elastic cons tants as a function of pressure, and the structural relaxation at vari ous pressure provide evidence for spontaneous elastic deformation, i.e ., a ferroelastic phase transition at 64 GPa of stishovite in the tetr agonal rutile structure to the orthorhombic CaCl2 structure. The on-si te and interatomic force constants are found to be consistent with the predicted structural phase transition. The dielectric properties and the phonon frequencies at the Gamma point are also computed in the CaC l2 structure.