Energetics of oxygen-octahedra rotations in perovskite oxides from first principles

TL;DR

This study uses first-principles calculations to analyze oxygen-octahedra rotations in perovskite oxides, explaining the stability of various tilt patterns and the dominant Pbnm phase, with implications for materials design.

Contribution

It provides a detailed analytical and numerical investigation of tilt pattern stability in perovskite oxides, clarifying the mechanisms behind the prevalence of the Pbnm phase and the role of secondary modes.

Findings

The Pbnm phase results from competing tilt patterns that coexist.

Strain effects are not crucial for tilt stability in these materials.

Secondary antipolar modes influence tilt competition and coexistence.

Abstract

We use first-principles methods to study oxygen-octahedra rotations in ABO3 perovskite oxides. We focus on the short-period, perfectly antiphase or in-phase, tilt patterns that characterize most compounds and control their physical (e.g., conductive, magnetic) properties. Based on an analytical form of the relevant potential energy surface, we discuss the conditions for the stability of polymorphs presenting different tilt patterns, and obtain numerical results for a collection of thirty-five representative materials. Our results reveal the mechanisms responsible for the frequent occurrence of a particular structure that combines antiphase and in-phase rotations, i.e., the orthorhombic Pbnm phase displayed by about half of all perovskite oxides and by many non-oxidic perovskites. The Pbnm phase benefits from the simultaneous occurrence of antiphase and in-phase tilt patterns that compete with each other, but not as strongly as to be mutually exclusive. We also find that secondary antipolar modes, involving the A cations, contribute to weaken the competition between different tilts and play a key role in their coexistence. Our results thus confirm and better explain previous observations for particular compounds. Interestingly, we also find that strain effects, which are known to be a major factor governing phase competition in related (e.g., ferroelectric) perovskite oxides, play no essential role as regards the relative stability of different rotational polymorphs. Further, we discuss why the Pbnm structure stops being the ground state in two opposite limits, for large and small A cations, showing that very different effects become relevant in each case. Our work thus provides a comprehensive discussion on these all-important and abundant materials, which will be useful to better understand existing compounds as well as to identify new strategies for materials engineering.