Dynamic Investigations of Pressure-Induced Abnormal Phase Transitions in PbTiO₃

The effects of pressure on phonon modes of ferroelectric tetragonal P4mm and paraelectric cubic Pm$\overline{3}$m PbTiO₃ are systematically investigated by using first-principles simulations. The pressure-induced tetragonal-to-cubic and subsequent cubic-to-tetragonal phase transitions are the second-order transitions, which are different from the phase transitions induced by temperature [Phys. Rev. Lett. 25 (1970) 167]. As pressure increases, the lowest A₁ and E modes of the tetragonal phase become softer and converge to the F_1u mode of the cubic phase. As pressure further increases, the lowest F_1u mode first hardens and then softens again, and finally diverges into A₁ and E modes. The behaviors of optical phonon modes confirm the ferroelectric-to-paraelectric-to-ferroelectric phase transitions. (paper)