  Metal-to-insulator phase transitions in complex oxide thin films are exciting
phenomena which may be useful for device applications, but in many cases the
physical mechanism responsible for the transition is not fully understood. Here
we demonstrate that epitaxial strain generates local disproportionation of the
NiO6 octahedra, driven through changes in the oxygen stoichiometry, and that
this directly modifies the metal-to-insulator phase transition in epitaxial
(001) NdNiO3 thin films. Theoretically, we predict that the Ni-O-Ni bond angle
decreases, while octahedral tilt and local disproportionation of the NiO6
octahedra increases resulting in a small band gap in otherwise metallic system.
This is driven by an increase in oxygen vacancy concentration in the rare-earth
nickelates with increasing in-plane biaxial tensile strain. Experimentally, we
find an increase in pseudocubic unit-cell volume and resistivity with
increasing biaxial tensile strain, corroborating our theoretical predictions.
With electron energy loss spectroscopy and x-ray absorption, we find a
reduction of the Ni valence with increasing tensile strain. These results
indicate that epitaxial strain modifies the oxygen stoichiometry of rare-earth
perovskite thin films and through this mechanism affect the metal-to-insulator
phase transition in these compounds.
