  Nickel-based superconductors provide a long-awaited experimental platform to
explore possible cuprate-like superconductivity. Despite similar crystal
structure and $d$ electron filling, these systems exhibit several differences.
Nickelates are the most polar layered oxide superconductor, raising questions
about the interface between substrate and thin film -- thus far the only sample
geometry to successfully stabilize superconductivity. We conduct a detailed
experimental and theoretical study of the prototypical interface between
Nd$_{1-x}$Sr$_x$NiO$_2$ and SrTiO$_3$. Atomic-resolution electron energy loss
spectroscopy in the scanning transmission electron microscope reveals the
formation of a single intermediate Nd(Ti,Ni)O$_3$ layer. Density functional
theory calculations with a Hubbard $U$ term show how the observed structure
alleviates the strong polar discontinuity. We explore effects of oxygen
occupancy, hole doping, and cation structure to disentangle the contributions
of each for reducing interface charge density. Resolving the nontrivial
interface structure will be instructive for future synthesis of nickelate films
on other substrates and in vertical heterostructures.
