Nickelate

Nickelate is a term used in inorganic and solid-state chemistry to describe compounds that contain nickel coordinated to oxide or nickel-oxide–based frameworks. In salts, nickelates may refer to compounds containing nickel-oxide networks or nickelate groups, such as layered oxides where nickel resides in octahedral coordination within NiO2 layers separated by alkali or other cations.

In solid-state chemistry, nickelates commonly denote materials with nickel-oxygen frameworks, notably the rare-earth nickelates with the general formula RNiO3 (R = a lanthanide). These compounds crystallize in distorted perovskite structures and feature nickel in the +3 oxidation state. They can exhibit a temperature- or pressure-driven metal–insulator transition and show strong electron correlation effects. The electronic properties can be tuned by substituting the rare-earth ion or by chemical pressure, and synthesis typically involves high-temperature solid-state reactions.

A distinct subset of nickelates, known as infinite-layer nickelates, has attracted attention for potential superconductivity. Examples such as Nd0.8Sr0.2NiO2 are prepared by reducing layered perovskites to remove apical oxide ions, yielding NiO2 planes with a formal Ni oxidation state near +1. Superconducting behavior has been reported in thin films and doped samples, though the mechanism remains under investigation and the materials are chemically sensitive.

Nickelates are studied for their electronic structure, phase behavior, and potential applications in catalysis, energy storage, and superconductivity. The term nickelate can also refer more broadly to nickel–oxygen species in solution or to nickel-containing oxide motifs in solid-state chemistry.