Ground-state valency and spin configuration of the nickelates.

The ab initio self-interaction-corrected local-spin-density approximation is used to study the electronic structure of both stoichiometric and non-stoichiometric nickelates. From total energy considerations it emerges that, in their ground-state, both LiNiO$_2$, and NaNiO$_2$ are insulators, with the Ni ion in the Ni$^{3+}$ low spin state ($t_{2g}^6e_{g}^1$) configuration. We find that the substitution of Li/Na atoms by divalent impurities, drives an equivalent number of Ni ions in the NiO$_2$ layers from the JT-active trivalent low-spin state to the divalent JT-inactive state. We propose that an experimental study on Mg$_x$Na$_{1-x}$NiO$_2$ might clarify the role of Ni$^{2+}$ impurities with respect to the vanishing of long range orbital ordering in Li$_{1-x}$Ni$_{1+x}$O$_2$. (Work sponsored by the Laboratory Directed Research and Development Program (LDRD) program of ORNL (LP, GMS, TE), and by the DOE-OS through the Offices of Basic Energy Sciences (BES), Division of Materials Sciences and Engineering (LP, GMS, TE). Oak Ridge National Laboratory is managed by UT-Battelle, LLC, for the Department of Energy under Contract No. DE-AC05-00OR22725.)