Pressure-induced instabilities in bulk TiO2 rutile

Density functional calculations, within the local density approximation, are used to investigate the isotropic pressure dependence of the geometry and Gamma-point phonons of TiO2 rutile over the range -8.6-8.6 GPa. The TO A(2u) mode, which is the c-axis ferroelectric mode, vanishes at P similar to -4 GPa, thereby leading to a crystal instability and a possible ferroelectric phase transition. The effects of a uniaxial strain along the c-axis on the geometry and the TO A(2u) mode are also investigated. This mode vanishes when the lattice parameter along c is just over 3% larger than in the unstrained case, again leading to a ferroelectric phase transition. Based on this result, it is suggested that expanded rutile structures might be created with enhanced dielectric properties by, for instance, thin film growth on a substrate with a small lattice mismatch. The microscopic origin of the ferroelectric stabilization is investigated and shows similarities with the case of perovskite oxides. The Raman-active B-1g mode unusually softens as the pressure increases and the atomistic origin of this behaviour is explained. The critical pressure for the second-order phase transition to a CaCl2-type structure induced by this softening (when combined with an orthorhombic distortion) is calculated to be 13 GPa. The results for the pressure dependence of the geometry and lattice dynamics agree well with the available measured data.