Emergence of long-range order in BaTiO3 from local symmetry-breaking distortions

TL;DR

This study reveals that local Ti displacements in BaTiO3 are zone-centered and rhombohedral-like across phases, and demonstrates how phase transitions emerge from local symmetry-breaking distortions, supporting an order-disorder model.

Contribution

The paper introduces a symmetry-based analysis of local distortions in BaTiO3 and develops a Monte Carlo model to explain its phase diagram from local correlations.

Findings

Ti displacements are zone-centered and rhombohedral-like across phases

Phase transitions can be explained by correlations of local distortions

Supports the order-disorder model for BaTiO3 transitions

Abstract

By using a symmetry motivated basis to evaluate local distortions against pair distribution function data (PDF), we show without prior bias, that the off-centre Ti displacements in the archetypal ferroelectric BaTiO3 are zone centred and rhombohedral-like in nature across its known ferroelectric and paraelectric phases. With our newly-gained insight we construct a simple Monte Carlo (MC) model which captures our main experimental findings and demonstrate how the rich crystallographic phase diagram of BaTiO3 emerges from correlations of local symmetry-breaking distortions alone. Our results strongly support the order-disorder picture for these phase transitions, but can also be reconciled with the soft-mode theory of BaTiO3 that is supported by some spectroscopic techniques.