First-Principles Study Of The Structural Instabilities In Hexagonal Barium Titanate: Coupling Between The Soft Optical And The Acoustic Modes

Details First-Principles Study Of The Structural Instabilities In Hexagonal Barium Titanate: Coupling Between The Soft Optical And The Acoustic Modes First-Principles Study Of The Structural Instabilities In Hexagonal Barium Titanate: Coupling Between The Soft Optical And The Acoustic Modes

1999-07-23

arxiv.org/abs/cond-mat/9907369v1

Physics

Condensed Matter

Materials Science

8 pages including 2 postscript figures, LaTeX with one special macro. Presented at the 9th. European Meeting on Ferroelectrici

Scientific paper

Hexagonal BaTiO_3 undergoes a structural phase transition to an orthorhombic C222_1 phase at T_0 = 222 K. The transition is driven by a soft optical mode with E_2u symmetry whose couplings force the appearance of a spontaneous E_2g strain (improper ferroelastic character). Staying within the same E_2u subspace, the system could in principle settle into a second (not observed) orthorhombic phase (Cmc2_1). We have carried out a first-principles investigation of these questions, studying the structure of the existing C222_1 and the "virtual" Cmc2_1 phases, and describing the spontaneous E_2g strain in accord with the experimental observations. In addition, we show that the occurrence of C222_1 instead of Cmc2_1 cannot be explained by the E_2u soft modes themselves and, therefore, must be related to their couplings with secondary order parameters. A more detailed analysis proves that the E_2g strains do not account for the experimental preference.

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