Xavier Gonze

The pressure-induced ferroelastic phase transition of SiO2 stishovite

Using a newly developed accurate ab initio technique, we study the stability and the lattice dynamics of SiO2 polymorph stishovite as a function of pressure. We observe a ferroelastic phase transition to CaCl2 structure at 64 GPa, driven by a mechanical instability associated with a vanishing shear modulus. Other lattice instabilities due to phonon anomalies are theoretically excluded along with a proposed softening of the B1g phonon mode.

Role of the Coulomb interaction on the ferroelectric instability of Barium Titanate

Abstract Born effective charges and dielectric tensor of barium titanate are obtained from first-principles calculations. The nature of large effective charges and their influence on the structural instability responsible of the ferroelectric phase transitions are discussed.