Charge-spin-orbital states in the tri-layered nickelate La4Ni3O8: an ab initio study

The electronic and magnetic structures of La4Ni3O8, an analogue of the hole doped cuprates, are studied using the configuration state constrained local-spin-density approximation plus Hubbard U calculations. It is found to be a C-type antiferromagnetic Mott insulator, in which an orbital hybridization strongly reduces an otherwise possible charge disproportionation. This state accounts for several experimental observations. The involved Ni2+ high-spin state and its orbital configuration are found to be against a crystal-field level picture, which predicts an Ni2+ low-spin state in the NiO2 square lattice. We note, however, that La4Ni3O8, if in the low-spin state, would be a charge-homogeneous ferromagnetic half-metal with only the up-spin x(2)-y(2) conduction band. Therefore, low-spin nickelates may be explored for any interesting property.