Negative pressure induced ferroelectric phase transition in rutile TiO₂

First-principles pseudopotential calculations by means of the local density approximation (LDA) within density-functional theory (DFT) are carried out to investigate the negative pressure induced ferroelectric phase transition in rutile TiO₂ in the range of −25 to 25 GPa. The softening behavior of the A_2u(TO) modes at the Γ point following the decreasing pressure leads to a ferroelectric phase transition from P4₂/mnm (rutile) space group to P42nm (ferro) space group. The calculated pressure dependence of the phonon frequencies, A₁(TO), E(TO), and B₂ modes of the ferro TiO₂ relative to A_2u(TO), E_u(TO), and B_1u modes of rutile TiO₂, indicates that the phase transition occurs at around −10 GPa, consistent with the total energy results. The dramatic increase of the c-axial dielectric tensor in the vicinity of the phase transition indicates it to be a typical ferroelectric phase transition. The character of the phase transition is generally identified in terms of the calculated order parameters, displaying a second order.