Abstract
The oxygen 1s absorption spectrum of La 2NiO 4 is very anisotropic. The ligand field splitting of the Ni 3d 9 upper Hubbard band in x 2 − y 2 and 3 z 2 − r 2 states is observed. Excess oxygen induces holes with primarily oxygen 2p character in this charge transfer insulator. These show up as a prepeak, approximately the band gap below the 3d 9 final states. Surprisingly, the intensity of the prepeak is isotropic. A plausible explanation is that the oxygen holes form a small polaron, removing the tetragonal distortion around a Ni-ion, and making all six NiO distances of equal length.
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