Infinite-layer LaNiO2:: Ni1+ is not Cu2+ -: art. no. 165109

The Ni ion in LaNiO2 has the same formal ionic configuration 3d(9) as does Cu in isostructural CaCuO2, but it is reported to be nonmagnetic and probably metallic whereas CaCuO2 is a magnetic insulator. From ab initio calculations we trace its individualistic behavior to (1) reduced 3d-2p mixing due to an increase of the separation of site energies (epsilon(d)-epsilon(p)) of at least 2 eV, and (2) important Ni 3d(3z(2)-r(2)) mixing with La 5d(3z(2)-r(2)) states that leads to Fermi surface pockets of La 5d character that hole dope the Ni 3d band. Correlation effects do not appear to be large in LaNiO2. However, ad hoc increase of the intra-atomic repulsion on the Ni site (using the LDA+U method) is found to lead to a correlated state: (i) the transition metal d(x(2)-y(2)) and d(3z(2)-r(2)) states undergo consecutive Mott transitions; (ii) their moments are antialigned leading (ideally) to a singlet ion in which there are two polarized orbitals; and (iii) mixing of the upper Hubbard 3d(3z(2)-r(2)) band with the La 5d(xy) states leaves considerable transition metal 3d character in a band pinned to the Fermi level. The magnetic configuration is more indicative of a Ni2+ ion in this limit, although the actual charge changes little with U.