  Epitaxial strain is a proven route to enhancing the properties of complex
oxides, however, the details of how the atomic structure accommodates strain
are poorly understood due to the difficulty of measuring the oxygen positions
in thin films. We present a general methodology for determining the atomic
structure of strained oxide films via x-ray diffraction, which we demonstrate
using LaNiO3 films. The oxygen octahedral rotations and distortions have been
quantified by comparing the intensities of half-order Bragg peaks, arising from
the two unit cell periodicity of the octahedral rotations, with the calculated
structure factor. Combining ab initio density functional calculations with
these experimental results, we determine systematically how strain modifies the
atomic structure of this functional oxide.
