Character of Holes inLixNi1−xOand Their Magnetic Behavior

Using oxygen $K$-edge x-ray-absorption spectroscopy we show that the holes compensating the ${\mathrm{Li}}^{1+}$ impurity charge in ${\mathrm{Li}}_x{\mathrm{Ni}}_{1-x}\mathrm{O}$ are located primarily in the O $2p$ states rather than in the conventionally assumed Ni $3d$ states. These holes are bound to the ${\mathrm{Li}}^{1+}$ impurities forming states in the NiO band gap. We also argue that these O $2p$ holes have a very large antiferromagnetic exchange interaction with neighboring ${\mathrm{Ni}}^{2+}$ ($d^8$) spins, which makes them appear like low-spin ${\mathrm{Ni}}^{3+}$ states in macroscopic magnetic measurements. This is similar to what could be happening in the Cu-based high-$T_c$ superconductors.