nickelates

Nickelates are oxide compounds that contain nickel and oxygen and exhibit a range of oxidation states for nickel, most commonly Ni2+ and Ni3+. The term encompasses NiO as well as a family of nickel-containing perovskite-like oxides with the general formula RNiO3, where R is a trivalent rare-earth or yttrium ion, as well as layered nickelates and the so-called infinite-layer nickelates such as RNiO2. These materials can be prepared by different routes and show diverse structural and electronic behavior.

Crystal structures vary across the nickelates. NiO adopts a rock-salt structure with nickel in octahedral coordination. In RNiO3 perovskites, nickel sits in octahedra within a distorted framework, and the Ni-O-Ni angles and bond lengths strongly influence electronic properties. Many RNiO3 compounds undergo a temperature-dependent metal-insulator transition, with the transition temperature depending on the rare-earth ion; LaNiO3 remains metallic over a wide temperature range. Recent work on infinite-layer nickelates, formed by reducing RNiO3 to RNiO2, has highlighted a structurally simpler NiO2 plane motif.

Electronic and magnetic properties of nickelates are governed by strong electron correlations. The nickelates often exhibit charge disproportionation, varied magnetic ordering, and metal-insulator transitions. Doping or reduction to lower-dimensional phases can modify transport and magnetic behavior and, in some cases, induce superconductivity in certain infinite-layer nickelates, drawing comparisons with cuprate superconductors.

Synthesis approaches range from conventional high-temperature solid-state methods for NiO to more specialized routes for RNiO3 and its reduced products, including topotactic reductions to obtain RNiO2. Applications and research focus areas include oxide electronics, catalysis, and the study of correlated electron phenomena in transition-metal oxides.