Structural instabilities in Pervskites: ab-initio calculation of the dynamical properties of ferroelectric PbTiO3

Abstract

A first-principles study of the vibrational modes of PbTiO3 in the ferroelectrictetragonal 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 T 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 a T plays a role in the discussion of the possibility of this phase transition. However, no mechanical instabilities (soft modes) are found, either at T or at any of the other high-symmetry points of the BZ.