Effect of Cobalt Substitution on the Jahn-Teller Distortion of the NaNiO₂Layered Oxide

NaNiO2 exhibits a layered structure made of NiO2 slabs between which Na+ ions are inserted in octahedral environment. Trivalent nickel ions in the low spin state (t2 6e1) exhibits a Jahn-Teller effect which leads to a distortion of the NiO6 octahedra and hence to a macroscopic lattice distortion. The partial substitution of trivalent cobalt for nickel (NaNi1−yCoyO2) allows to suppress the structural distortion for y≥0.22. This behavior results from the competition between the stabilization of Ni3+ ions in distorted environment and the strong stabilization of the isotropic Co3+ ions in octahedral environment.