  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.
