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

The layered square-planar nickelates, Nd_n+1Ni_nO_2n+2, are an appealing system to tune the electronic properties of square-planar nickelates via dimensionality; indeed, superconductivity was recently observed in Nd₆Ni₅O₁₂ thin films. Here, we investigate the role of epitaxial strain in the competing requirements for the synthesis of the n = 3 Ruddlesden-Popper compound, Nd₄Ni₃O₁₀, and subsequent reduction to the square-planar phase, Nd₄Ni₃O₈. We synthesize our highest quality Nd₄Ni₃O₁₀ films under compressive strain on LaAlO₃ (001), while Nd₄Ni₃O₁₀ on NdGaO₃ (110) exhibits tensile strain-induced rock salt faults but retains bulk-like transport properties. A high density of extended defects forms in Nd₄Ni₃O₁₀ on SrTiO₃ (001). Films reduced on LaAlO₃ become insulating and form compressive strain-induced c-axis canting defects, while Nd₄Ni₃O₈ films on NdGaO₃ are metallic. This work provides a pathway to the synthesis of Nd_n+1Ni_nO_2n+2 thin films and sets limits on the ability to strain engineer these compounds via epitaxy.