Lattice effects on the formation of oxygen vacancies in perovskite thin films

TL;DR

This study uses first-principles calculations to show how lattice vibrations influence oxygen vacancy formation in perovskite thin films, revealing temperature-dependent effects and the impact on vacancy ordering.

Contribution

It demonstrates the significant role of phonons and thermal lattice effects on defect chemistry and vacancy formation trends in strained perovskite thin films, especially near room temperature.

Findings

Phonons reverse zero-temperature vacancy trends at room temperature.

Lattice expansion correlates with increased vacancy formation favorability.

Thermal excitations hinder vacancy ordering.

Abstract

We use first-principles methods to investigate the effects of collective lattice excitations on the formation of oxygen vacancies in perovskite thin films. We find that phonons play a crucial role on the strain-mediated control of defect chemistry at finite temperatures. In particular, zero-temperature oxygen vacancy formation trends deduced as a function of epitaxial strain can be fully reversed near room temperature. Our first-principles calculations evidence a direct link between the lattice contribution to the oxygen vacancy free energy and the volume expansion that the system undergoes when is chemically reduced: The larger the resulting volume expansion, the more favorable thermal excitations are to point defect formation. However, the interplay between the vibrational vacancy entropy, or equivalently, chemical expansion, and epitaxial strain is difficult to generalise as this can be strongly influenced by underlying structural and magnetic transitions. In addition, we find that vacancy ordering can be largely hindered by the thermal lattice excitations.