Low-lying unoccupied electronic states in3dtransition-metal fluorides probed by NEXAFS at theF1sthreshold

Abstract

The near-edge x-ray absorption fine structure (NEXAFS) at the $\mathrm{F}\phantom{\rule{0.3em}{0ex}}1s$ threshold has been studied with high-energy resolution for a series of binary fluorides, including KF, $\mathrm{Ti}{\mathrm{F}}_{4}$, $\mathrm{V}{\mathrm{F}}_{4}$, $\mathrm{V}{\mathrm{F}}_{3}$, $\mathrm{Cr}{\mathrm{F}}_{3}$, $\mathrm{Cr}{\mathrm{F}}_{2}$, $\mathrm{Mn}{\mathrm{F}}_{3}$, $\mathrm{Mn}{\mathrm{F}}_{2}$, $\mathrm{Fe}{\mathrm{F}}_{3}$, $\mathrm{Fe}{\mathrm{F}}_{2}$, $\mathrm{Co}{\mathrm{F}}_{2}$, $\mathrm{Ni}{\mathrm{F}}_{2}$, $\mathrm{Cu}{\mathrm{F}}_{2}$, and $\mathrm{Zn}{\mathrm{F}}_{2}$ as well as for $\mathrm{S}{\mathrm{F}}_{6}$ in the gas phase, and for the $\mathrm{P}{\mathrm{F}}_{6}^{\ensuremath{-}}$ and $\mathrm{Ti}{\mathrm{F}}_{6}^{2\ensuremath{-}}$ molecular anions of the solid compounds $\mathrm{K}\mathrm{P}{\mathrm{F}}_{6}$ and ${\mathrm{K}}_{2}\mathrm{Ti}{\mathrm{F}}_{6}$. Most of these spectra were measured at the Russian-German beamline at BESSY II, while the spectra of KF and $\mathrm{Cu}{\mathrm{F}}_{2}$ 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 $\mathrm{F}\phantom{\rule{0.3em}{0ex}}1s$ NEXAFS spectrum of the molecular $\mathrm{Ti}{\mathrm{F}}_{6}^{2\ensuremath{-}}$ anion in solid ${\mathrm{K}}_{2}\mathrm{Ti}{\mathrm{F}}_{6}$. In its turn, the latter spectrum has been interpreted by comparing with the $\mathrm{F}\phantom{\rule{0.3em}{0ex}}1s$ NEXAFS spectrum of the molecular $\mathrm{P}{\mathrm{F}}_{6}^{\ensuremath{-}}$ anion in $\mathrm{K}\mathrm{P}{\mathrm{F}}_{6}$ and that of $\mathrm{S}{\mathrm{F}}_{6}$ 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 $3d$\ensuremath{-}fluorine $2p$ hybridized character decreases gradually along the series of $3d$ TM fluorides, and is essentially zero in the case of $\mathrm{Zn}{\mathrm{F}}_{2}$.