  Ultrathin epitaxial films of EuNiO3 were grown on a series of substrates
traversing highly compressive (- 2.4%) to highly tensile (2.5%) lattice
mismatch. X-ray diffraction measurements showed the expected c-lattice
parameter shift for compressive strain, but no detectable shift for tensilely
strained substrates, while reciprocal space mapping confirmed the tensile
strained film maintained epitaxial coherence. Transport measurements showed a
successively (from tensile to compressive) lower resistance and a complete
suppression of the metalinsulator transition at highly compressive lattice
mismatch. Corroborating these findings, X-ray absorption spectroscopy
measurements revealed a strong multiplet splitting in the tensile samples that
progressively weakens with increasing compressive strain that, combined with
cluster calculations, showed enhanced covalence between Ni-d and O-p orbitals
leads to the metallic state.
