Electronic structure of K 2 NiF 4

An experimental and theoretical study of the electronic structure of ${\mathrm{K}}_{2}$${\mathrm{NiF}}_{4}$ is presented. Core-level and valence-band x-ray angle-integrated photoemission experiments are reported and the data analyzed through a configuration-interaction impurity-cluster model. The charge transfer and the Mott-Hubbard charge fluctuations are studied and the energies involved estimated. The effect of these mechanisms on the spectral structures is identified. The comparison of local-density-functional band calculations with angle-resolved photoemission experiments shows significant discrepancies. This demonstrates that electron-correlation effects are indeed important and one-electron band-structure calculations are inadequate to describe this compound.