Conductivity Properties of Perovskite Nickelates and Cuprates Depend on the Oxidation States of the Metal Ions

Superconducting nickelates in thin film perovskite structures, designed to be isostructural with the Cu(II) cuprates, have been discovered recently by Zhou et al. (Appl. Mater. Interfaces 10, 1463-1467, 1) and Li et al. (Nature 572, 624-627, 2) The strategies of both groups, to make nickelates as similar to the cuprates as possible by using planar NiO2 superlattice structures, proved successful. Ni(I) and Ni(III) are superconducting in superlattices, although TC is lower than in most Cu(II) cuprates. Here we find that a “3-oxidation states rule” applies to nickelate superlattices based on Ni(I) and Ni(III). It is predicted that the oxides of Ni(II), Cu(I), and Cu(III) cannot superconduct, because of violation of the 3-rule. This rule is derived here, along with other rules where the number of interacting oxidation states is decisive for the conduction properties of the metal ion. The Mott model is replaced by a free energy model.