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

TL;DR

This study uses first-principles calculations to analyze the structural phase transition in hexagonal BaTiO3, focusing on the coupling between soft optical modes and acoustic modes, and explaining the observed phase stability.

Contribution

It provides a detailed first-principles analysis of the coupling mechanisms driving the phase transition and explains why the observed phase is favored over a theoretically possible alternative.

Findings

The C222_1 phase is stabilized due to mode couplings, not just soft modes.

E_2g strains alone do not explain the phase preference.

The study models both existing and hypothetical phases to understand transition mechanisms.

Abstract

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.