Solid state reduction of nickelate thin films

The doped square-planar nickelates are a novel class of metal oxide superconductor. Their synthesis involves growth of a perovskite precursor followed by topotactic reduction. Current reduction processes involve annealing in vacuum with CaH₂ powder. This ex-situ process allows control of the oxygen vacancy content in the thin films but is not easily amenable to in-situ measurements of the dynamics of square-planar nickelate formation.

To address this challenge, we develop an in-situ process via the deposition of metallic aluminum in ultra-high vacuum (UHV), and we demonstrate the process by reducing NdNiO₃ thin films. By characterizing the NdNiO_3-x thin films dynamically during the reduction process, we show that the Ni valence and NdNiO_3-x lattice constant can be precisely controlled at each step of the Al deposition process, while maintaining the film’s epitaxy and surface morphology. We also apply this reduction method to Nd₆Ni₅O₁₆ films to synthesize Nd₆Ni₅O₁₂ films, a system for which superconductivity has been recently reported. The metal reduction method is straightforward and widely applicable for square-planar nickelate systems.