Hydrogen incorporation during reduction to the infinite-layer phase of superconducting nickelates

A recently discovered and expanding field, superconductivity in thin-film nickelates such as Nd_1-xSr_xNiO₂ (0.125 < x < 0.25) relies on the unusual Ni+ valence state stabilized by the infinite-layer structure. This structural phase and therefore superconductivity are, for complex reasons, difficult to synthesize and reproduce. Precursor films of the perovskite ANiO₃ undergo chemical reduction through reaction with CaH₂, but reduction may not be uniform through the depth of the film, and hydrogen may be incorporated into the film through the surface; either may from theory suppress superconductivity. We used the complementary depth-profiling techniques of neutron reflectometry and secondary ion mass spectroscopy to explore the role of cation doping, film thickness, sample preparation, and strain on uniformity of the infinite-layer phase in reduced ANiO₃ (A = Nd, Pr, Sr) nickelate thin films. We find that depending on these details, films can be composed of multiple regions of varying oxygen content, and hydrogen species are observed throughout the films in decreasing concentration away from the surface. These non-ideal infinite-layer phases correlate with lower crystalline quality and higher resistance. Thus, these variables are crucial to designing superconducting nickelate films.