| Summary: | The electronic and magnetic structures of La _4 Ni _3 O _8 , 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 Ni ^2+ high-spin state and its orbital configuration are found to be against a crystal-field level picture, which predicts an Ni ^2+ low-spin state in the NiO _2 square lattice. We note, however, that La _4 Ni _3 O _8 , 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.
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