EFFECT OF COBALT SUBSTITUTION ON THE JAHN-TELLER DISTORTION OF THE NANIO2 LAYERED OXIDE

NaNiO2 exhibits a layered structure made of NiO2 slabs between which N a+ ions are inserted in octahedral environment. Trivalent nickel ions in the low spin state (t(2)(6)e(1)) exhibits a Jahn-Teller effect whic h leads to a distortion of the NiO6 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 great er than or equal to 0.22. This behavior results from the competition b etween the stabilization of Ni3+ ions in distorted environment and the strong stabilization of the isotropic Co3+ ions in octahedral environ ment.