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

García, Alberto
; Vanderbilt, Davis

Publication year:

1997

Publication place:

Donostia-San Sebastián

ISSN:

1137-4411

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Summary

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 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.