Pressure-induced instabilities in bulkTiO2 rutile

Abstract

Density functional calculations, within the local density approximation, areused to investigate the isotropic pressure dependence of the geometry andΓ-point phononsof TiO2 rutile over therange −8.6–8.6 GPa.The TO A2u mode,which is the c-axis ferroelectric mode, vanishes at GPa, thereby leading to a crystal instability and a possibleferroelectric phase transition. The effects of a uniaxial strain along thec-axis on the geometryand the TO A2u mode are also investigated. This mode vanishes when the lattice parameter alongc is just over 3% larger than in the unstrained case, again leading to a ferroelectric phasetransition. Based on this result, it is suggested that expanded rutile structures might becreated with enhanced dielectric properties by, for instance, thin film growth on a substratewith a small lattice mismatch. The microscopic origin of the ferroelectric stabilization isinvestigated and shows similarities with the case of perovskite oxides. The Raman-activeB1g mode unusually softens as the pressure increases and the atomistic origin of thisbehaviour is explained. The critical pressure for the second-order phase transition to aCaCl2-type structure induced by this softening (when combined with an orthorhombicdistortion) is calculated to be 13 GPa. The results for the pressure dependence ofthe geometry and lattice dynamics agree well with the available measured data.