Hole-states in Li doped NiO: doping dependence of Zhang-Rice spectral weight

We report on structural evolution and a dynamical transfer of spectral weight as observed in O K-edge X-ray absorption (XA) spectra upon hole doping in LiNi_1-O (0 ≤ x ≤ 0.24). We find that the unit cell of doped NiO evolves in a specific way up to an intermediate doping level (x = 0.12) and then follows a different trend. A double peak structure (one is localized and the other is more delocalized) of low energy spectral weight (LESW) at the O K-edge is identified and quantified. Normalized LESW is found to increase linearly with an effective doping concentration (∼x/(1 - 2x)) with a change in slope at x = 0.12, which is correlated to the observed structural evolution. Doping dependence of Ni L₃-edge XA spectral shape closely follows the localized peak of LESW at the O K-edge. Our observations strongly support the Zhang-Rice (ZR) picture, confirming that doped holes in NiO are of primarily ZR character. Furthermore, we find that Fe co-doping destroys the ZR spectral weight and recovers the spectral shapes to that of NiO.