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 $S=1/2$ magnetic insulator be realized with the difficult ${\mathrm{Ni}}^{+}$ oxidation state and possibly doped with low spin $(S=0){\mathrm{Ni}}^{2+}$ 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 $S=1 {\mathrm{Ni}}^{2+}$ embedded in a sea of itinerant O holes. Strong coupling of these holes to the localized spins via $2p\ensuremath{-}3d$ hybridization leads to a heavy-fermion system with a large Kondo temperature, which was confirmed experimentally for ${\mathrm{LaNiO}}_{3}.$