  Controlling phase transitions in transition metal oxides remains a central
feature of both technological and fundamental scientific relevance. A
well-known example is the metal-insulator transition which has been shown to be
highly controllable while a less well understood aspect of this phenomenon is
the length scale over which the phases can be established. To gain further
insight into this issue, we have atomically engineered an artificially phase
separated system through fabricating epitaxial superlattices consisting of
SmNiO$_{3}$ and NdNiO$_{3}$, two materials undergoing a metal-to-insulator
transition at different temperatures. By combining advanced experimental
techniques and theoretical modeling, we demonstrate that the length scale of
the metal-insulator transition is controlled by the balance of the energy cost
of the domain wall between a metal and insulator and the bulk energetics.
Notably, we show that the length scale of this effect exceeds that of the
physical coupling of structural motifs, introducing a new paradigm for
interface-engineering properties that are not available in bulk
