First-principles study of stability and vibrational properties of tetragonal ${\mathrm{PbTiO}}_3$

A first-principles study of the vibrational modes of ${\mathrm{PbTiO}}_3$ in the ferroelectric tetragonal phase has been performed at all the main symmetry points of the Brillouin zone (BZ). The calculations use the local-density approximation and ultrasoft pseudopotentials with a plane-wave basis, and reproduce well the available experimental information on the modes at the Γ point, including the LO-TO splittings. The work was motivated in part by a previously reported transition to an orthorhombic phase at low temperatures [(J. Kobayashi, Y. Uesu, and Y. Sakemi, Phys. Rev. B 28, 3866 (1983).] We show that a linear coupling of orthorhombic strain to one of the modes at Γ plays a role in the discussion of the possibility of this phase transition. However, no mechanical instabilities (soft modes) are found, either at Γ or at any of the other high-symmetry points of the BZ. © 1996 The American Physical Society.