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

Abstract: The layered square-planar nickelates, Nd${n+1}$Ni${n}$O${2n+2}$, are an appealing system to tune the electronic properties of square-planar nickelates via dimensionality; indeed, superconductivity was recently observed in Nd${6}$Ni${5}$O${12}$ thin films. Here, we investigate the role of epitaxial strain in the competing requirements for the synthesis of the $n=3$ Ruddlesden-Popper compound, Nd${4}$Ni${3}$O${10}$, and subsequent reduction to the square-planar phase, Nd${4}$Ni${3}$O${8}$. We synthesize our highest quality Nd${4}$Ni${3}$O${10}$ films under compressive strain on LaAlO${3}$ (001), while Nd${4}$Ni${3}$O${10}$ on NdGaO${3}$ (110) exhibits tensile strain-induced rock salt faults but retains bulk-like transport properties. A high density of extended defects forms in Nd${4}$Ni${3}$O${10}$ on SrTiO${3}$ (001). Films reduced on LaAlO${3}$ become insulating and form compressive strain-induced $c$-axis canting defects, while Nd${4}$Ni${3}$O${8}$ films on NdGaO${3}$ are metallic. This work provides a pathway to the synthesis of Nd${n+1}$Ni${n}$O${2n+2}$ thin films and sets limits on the ability to strain engineer these compounds via epitaxy.