Low-lying unoccupied electronic states in 3d transition-metal fluorides probed by NEXAFS at the F 1s threshold

The near-edge x-ray absorption fine structure (NEXAFS) at the F 1s threshold has been studied with high-energy resolution for a series of binary fluorides, including KF, TiF₄, VF₄, VF₃, CrF₃, CrF₂, MnF₃, MnF₂, FeF₃, FeF₂, CoF₂, NiF₂, CuF₂, and ZnF₂ as well as for SF₆ in the gas phase, and for the PF₆⁻ and TiF₆²⁻ molecular anions of the solid compounds KPF₆ and K₂TiF₆. Most of these spectra were measured at the Russian-German beamline at BESSY II, while the spectra of KF and CuF₂ were taken under comparable experimental conditions at the D1011 beamline at MAX-lab. The spectra of the solid samples were recorded via the total electron yield. The NEXAFS spectra were taken with the aim to elucidate the role of covalent bonding and its manifestation in x-ray absorption spectra as well as to gain information on the electronic structure of the conduction band along the whole series of 3d transition-metal (TM) fluorides. The spectra of these most ionic compounds of the 3d TM’s have been analyzed in a comparative way considering also the F 1s NEXAFS spectrum of the molecular TiF₆²⁻ anion in solid K₂TiF₆. In its turn, the latter spectrum has been interpreted by comparing with the F 1s NEXAFS spectrum of the molecular PF₆⁻ anion in KPF₆ and that of SF₆ in the gas phase. In this way, the low-lying empty electronic states of the 3d TM fluorides are shown to be formed by covalent mixing of the TM 3d with the fluorine 2p electronic states. It is further found that the number of low-lying empty electronic states with TM 3d−fluorine 2p hybridized character decreases gradually along the series of 3d TM fluorides, and is essentially zero in the case of ZnF₂.