Lattice Effects on the Formation of Oxygen Vacancies in Perovskite Thin Films

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 in the strain-mediated control of defect chemistry at finite temperatures. Specifically, 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 it 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 generalize, as it 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.