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 TiO(2) 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 P4(2)/mnm (rutile) space group to P42nm (ferro) space group. The calculated pressure dependence of the phonon frequencies, A(1)(TO), E(TO), and B(2) modes of the ferro TiO(2) relative to A(2u)(TO), E(u)(TO), and B(1u) modes of rutile TiO(2), 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.