  Oxygen-defect control has long been considered an influential tuning knob for
producing various property responses in complex oxide films. In addition to
physical property changes, modification to the lattice structure, specifically
lattice expansion, with increasing oxygen vacancy concentrations has been
reported often and has become the convention for oxide materials. However, the
current understanding of the lattice behavior in oxygen-deficient films becomes
disputable when considering compounds containing different bonding environments
or atomic layering. Moreover, tensile strain has recently been discovered to
stabilize oxygen vacancies in epitaxial films, which further complicates the
interpretation of lattice behavior resulting from their appearance. Here, we
report on the selective strain control of oxygen vacancy formation and
resulting lattice responses in the layered, Ruddlesden-Popper phases,
La1.85Sr0.15CuO4. We found that a drastically reduced Gibbs free energy for
oxygen vacancy formation near the typical growth temperature for
tensile-strained epitaxial LSCO accounts for the large oxygen
non-stoichiometry. Additionally, oxygen vacancies form preferentially in the
equatorial position of the CuO2 plane, leading to a lattice contraction, rather
than the expected expansion, observed with apical oxygen vacancies. Since
oxygen stoichiometry plays a key role in determining the physical properties of
many complex oxides, the strong strain coupling of oxygen nonstoichiometry and
the unusual structural response reported here can provide new perspectives and
understanding to the structure and property relationships of many other
functional oxide materials.
