From: David Vanderbilt <dhv@physics.rutgers.edu> Date: Thu, 14 Mar 1996 11:37:50 -0500
Author(s): Alberto Garcia (Departmento de Fisica Aplicada II, Universidad del Pais Vasco, Bilbao, Spain) , David Vanderbilt (Department of Physics, Astronomy, Rutgers University)
Comments: 8 pages, two-column style with 3 postscript figures embedded. Uses REVTEX and epsf macros. Also available at
this http URL
A first-principles study of the vibrational modes of 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 Gamma 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 {\bf 28}, 3866 (1983)]. We show that a linear coupling of orthorhombic strain to one of the modes at Gamma plays a role in the discussion of the possibility of this phase transition. However, no mechanical instabilities (soft modes) are found, either at Gamma or at any of the other high-symmetry points of the BZ.