Electronic structure of possible nickelate analogs to the cuprates

The electronic structure of various nickel oxides with nickel valence varying from 1+ to 3+ was investigated with the aim to find similarities and differences to the isoelectronic cuprates. Only if the Ni ions are forced into a planar coordination with the O ions can a $S=1/2\mathrm{}$ magnetic insulator be realized with the difficult ${\mathrm{Ni}}^{+}$ oxidation state and possibly doped with low spin $\mathrm{}(S=0\mathrm{})\mathrm{}{\mathrm{Ni}}^{2+}$ holes directly analogous to the superconducting cuprates. The more common ${\mathrm{Ni}}^{3+}$ oxidation state cannot be used to make a parent magnetic insulator as it forms rather as localized $S=1\mathrm{}\hspace{0.5em}{\mathrm{Ni}}^{2+}$ embedded in a sea of itinerant O holes. Strong coupling of these holes to the localized spins via $2p-3d$ hybridization leads to a heavy-fermion system with a large Kondo temperature, which was confirmed experimentally for ${\mathrm{LaNiO}}_3.$