  Bulk NdNiO3 exhibits a metal-to-insulator transition (MIT) as the temperature
is lowered that is also seen in tensile strained films. In contrast, films that
are under a large compressive strain typically remain metallic at all
temperatures. To clarify the microscopic origins of this behavior, we use
position averaged convergent beam electron diffraction in scanning transmission
electron microscopy to characterize strained NdNiO3 films both above and below
the MIT temperature. We show that a symmetry lowering structural change takes
place in case of the tensile strained film, which undergoes an MIT, but is
absent in the compressively strained film. Using space group symmetry
arguments, we show that these results support the bond length
disproportionation model of the MIT in the rare-earth nickelates. Furthermore,
the results provide insights into the non-Fermi liquid phase that is observed
in films for which the MIT is absent.
