First-principles study of stability and vibrational properties of tetragonal PbTiO3.

A first-principles study of the vibrational modes of PbTiO3 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 G 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 G plays a role in the discussion of the possibility of this phase transition. However, no mechanical instabilities ~soft modes! are found, either at G or at any of the other high-symmetry points of the BZ. @S0163-1829~96!04529-8#