First-principles study of ferroelectric and antiferrodistortive instabilities in tetragonal SrTiO3

Details First-principles study of ferroelectric and antiferrodistortive instabilities in tetragonal SrTiO3 First-principles study of ferroelectric and antiferrodistortive instabilities in tetragonal SrTiO3

2000-04-21

arxiv.org/abs/cond-mat/0004372v1

Phys. Rev. B 62, 13942 (2000)

Physics

Condensed Matter

Materials Science

10 pages, two-column style with 4 postscript figures embedded. Uses REVTEX and epsf macros. Also available at http://www.phy

Scientific paper

10.1103/PhysRevB.62.13942

We carry out first-principles density-functional calculations of the antiferrodistortive (AFD) and ferroelectric (FE) soft-mode instabilities in tetragonal SrTiO3, with the structural degrees of freedom treated in a classical, zero-temperature framework. In particular, we use frozen-phonon calculations to make a careful study of the anisotropy of the AFD and FE mode frequencies in the tetragonal ground state, in which an R-point AFD soft phonon has condensed. Because of the anharmonic couplings, the presence of this AFD distortion substantially affects both the AFD and FE mode frequencies. The AFD mode is found to be softer for rotations around a perpendicular axis (Eg mode) than for rotations about the tetragonal axis (A1g mode), in agreement with experimental results. The FE mode, on the other hand, is found to be softer when polarized perpendicular to the tetragonal axis (Eu mode) than parallel to it (A2u mode). The sign of this frequency splitting is consistent with the experimentally reported anisotropy of the dielectric susceptibility and other evidence. Finally, we present a discussion of the influence of various types of structural distortions on the FE instability and its anisotropy.

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