  Heteroepitaxy offers a new type of control mechanism for the crystal
structure, the electronic correlations, and thus the functional properties of
transition-metal oxides. Here, we combine electrical transport measurements,
high-resolution scanning transmission electron microscopy (STEM), and density
functional theory (DFT) to investigate the evolution of the metal-to-insulator
transition (MIT) in NdNiO$_3$ films as a function of film thickness and
NdGaO$_3$ substrate crystallographic orientation. We find that for two
different substrate facets, orthorhombic (101) and (011), modifications of the
NiO$_6$ octahedral network are key for tuning the transition temperature
$T_{\text{MIT}}$ over a wide temperature range. A comparison of films of
identical thickness reveals that growth on [101]-oriented substrates generally
results in a higher $T_{\text{MIT}}$, which can be attributed to an enhanced
bond-disproportionation as revealed by the DFT+$U$ calculations, and a tendency
of [011]-oriented films to formation of structural defects and stabilization of
non-equilibrium phases. Our results provide insights into the
structure-property relationship of a correlated electron system and its
evolution at microscopic length scales and give new perspectives for the
epitaxial control of macroscopic phases in metal-oxide heterostructures.
