Oxygen disorder, a way to accommodate large tensile strains in oxide thin films

Strain induced by lattice mismatch of epitaxial layers is typically accommodated by misfit dislocations. In transition-metal oxides, other strain-relaxation modes have been observed (oxygen vacancy, octahedral tilts, etc.). Here we use density functional calculations to compare the total energies of different structures and to check for negative-frequency phonon modes, which is an good indicator of instability, and explore the stability of several oxide thin films. We find that when a ZrO2 thin film is sandwiched between SrTiO3 layers (7{\%} biaxial tensile strain), disorder in the oxygen sublattice lowers the energy by 1.4 eV/ZrO2 formula and leads to a stable configuration without negative-frequency phonon modes. Oxygen disorder can also accommodate a 6{\%} biaxial tensile strain in rutile TiO2 thin film. In contrast, we find that if a (LaFeO3)2/(SrFeO3) superlattice is grown on a substrate that imposes an overall biaxial tensile strain, there is a competition between O sublattice disorder, formation and ordering of O vacancies, and octahedral tilts. The mechanism for strain compensation varies with the extent of the strain. We conclude that oxygen-sublattice disorder is one of many ways that tensile strain can be accommodated in transition-metal oxide films.