Oxygen ordered phases and resonant scattering measurements of the infinite-layer nickelates

Materials hosting unconventional and high T_c superconductivity demonstrate complex phase diagrams where superconductivity cooperates and competes with other ground states involving entangled charge, lattice, and spin degrees of freedom. The infinite-layer nickelates, RENiO₂, RE=La, Nd, Pr, offer a novel platform to study the nature of high T_c superconductivity and the normal state from which it emerges – possibly allowing discernment of the universal elements of the high T_c phase diagram. While the normal state of the infinite-layer nickelates shows noticeable differences from that of the cuprates (lacking a long-range ordered AFM insulating phase), recent work has shed light on some of the similarities, including the presence of magnetic fluctuations and a purported charge ordered phase in the undoped material. However, this class of materials poses significant synthesis challenges which must be overcome to reveal their intrinsic ground state properties. Here, we discuss the synthesis of perovskite and infinite-layer nickelates by reactive oxide molecular beam epitaxy and activated hydrogen reduction. We then reexamine the presence of charge density wave order in NdNiO₂ and, using a multimodal approach, show that the previously reported 3a₀ ordering can be attributed to interstitial oxygen ordering rather than a correlation driven density wave.