Unoccupied density of states of La2–xSrxNiO4+δ studied by polarization-dependent x-ray-absorption spectroscopy and bremsstrahlung isochromat spectroscopy

Oxygen 1s x-ray-absorption measurements of La2–xSrxNiO4+δ (0≤x≤1.15) are presented, together with O 1s polarization-dependent x-ray absorption on single-crystal La1.85Sr0.15NiO4. It is concluded that the charge-compensating holes have mainly oxygen 2p character. The Ni 3d9 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 px,py and out-of-plane pz orbitals. Possible explanations for this are presented. The intrinsic band gap of La2NiO4 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