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  behavior is only observed in the infinite layer A₁B₁O₂ (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 such as CaH₂. However, the topotactic phase transition is non-trivial and difficult to control as it is correlated with the significant generation of defects (oxygen vacancies) throughout the entirety of the film. Here we present results from in situ synchrotron X-ray studies of the reduction reaction in nickelate heterostructures. The films are grown by pulsed laser deposition from a Nd_0.8Sr_0.2NiO₃ target on SrTiO₃ (001). We will discuss the evolution of the Nd_0.8Sr_0.2NiO₃ structure as a function of reduction temperature and time. We find that the gas-solid reaction with CaH₂ leaves both the infinite-layer Nd_0.8Sr_0.2NiO₂ and an intermediate Nd_0.8Sr_0.2NiO_2+δ phase, accompanied with changes to the oxygen octahedral rotation pattern from a^-a^-c^- to a⁰a⁰c^-. Our results provide much needed structural insight into the 113 to 112 phase transition in Nd_0.8Sr_0.2NiO₃ and other complex oxide heterostructures.