In Situ Synchrotron X-ray Studies of Nickelate Growth and Reduction*

Long-sought non-cuprate superconductivity has recently been discovered in epitaxial thin films of alloyed neodymium nickelate (Nd_0.8Sr_0.2NiO₂) [1]. Interestingly, the superconducting phase is only observed in the infinite layer A₁B₁O₂ or ‘112’ phase, and not the more typical A₁B₁O₃ (‘113’) perovskite phase. Reduction of the 113 to the 112 phase generally requires use of a strong reducing agent like hydrides. However, the topotactic transition process involving significant oxygen loss is intrinsically non-trivial and remains elusive for precise synthesis control. Here we present the results of in situ synchrotron X-ray studies on the growth of nickelate heterostructures by pulsed laser deposition (PLD). We employ a Nd_0.8Sr_0.2NiO₃ target for the growth of Nd_0.8Sr_0.2NiO₃ / SrTiO₃ (001) heterostructures and NdNiO₃ and SrNiO_x targets for the growth of nickelate superlattices. We will discuss the relationships between phase stability and the oxygen concentration for different deposition parameters prior to CaH₂ reduction. The results of in situ studies of the reduction process to realize square-planar nickelates will also be described.