Limits to the strain engineering of layered square-planar nickelate thin films.

The layered square-planar nickelates, NdNiO, are an appealing system to tune the electronic properties of square-planar nickelates via dimensionality; indeed, superconductivity was recently observed in NdNiO thin films. Here, we investigate the role of epitaxial strain in the competing requirements for the synthesis of the n = 3 Ruddlesden-Popper compound, NdNiO, and subsequent reduction to the square-planar phase, NdNiO. We synthesize our highest quality NdNiO films under compressive strain on LaAlO (001), while NdNiO on NdGaO (110) exhibits tensile strain-induced rock salt faults but retains bulk-like transport properties. A high density of extended defects forms in NdNiO on SrTiO (001). Films reduced on LaAlO become insulating and form compressive strain-induced c-axis canting defects, while NdNiO films on NdGaO are metallic. This work provides a pathway to the synthesis of NdNiO thin films and sets limits on the ability to strain engineer these compounds via epitaxy.