Unoccupied density of states of ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{NiO}}_{4+\mathrm{\delta }}$ studied by polarization-dependent x-ray-absorption spectroscopy and bremsstrahlung isochromat spectroscopy

Oxygen 1s x-ray-absorption measurements of ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$ ${\mathrm{Sr}}_{\mathit{x}}$ ${\mathrm{NiO}}_{4+\mathrm{\delta }}$ (0≤x≤1.15) are presented, together with O 1s polarization-dependent x-ray absorption on single-crystal ${\mathrm{La}}_{1.85}$ ${\mathrm{Sr}}_{0.15}$ ${\mathrm{NiO}}_4$. It is concluded that the charge-compensating holes have mainly oxygen 2p character. The Ni 3${\mathit{d}}^9$ final states reached because of the p-d hybridization in the ground state are strongly polarized in the (a,b) plane. The Sr-induced O 2p holes are found to be distributed equally over in-plane ${\mathit{p}}_{\mathit{x}}$,${\mathit{p}}_{\mathit{y}}$ and out-of-plane ${\mathit{p}}_{\mathit{z}}$ orbitals. Possible explanations for this are presented. The intrinsic band gap of ${\mathrm{La}}_2$ ${\mathrm{NiO}}_4$ is found to be at least 4 eV. The bremsstrahlung-isochromat-spectroscopy results show that the Sr-induced states are spread out over a large portion of the band gap and are not concentrated at the top of the valence band.