Near edge absorption studies of pure and impure NbSe2; theory and experiment.

In this study, we present X-ray absorption near edge structure (XANES) for four different layered transition metal dichalcogenide compounds, (i) pure NbSe 2 , (ii) NbSe 2 with Se vacancy, (iii) impure NbSe 2 with Cr defects, and (iv) impure NbSe 2 with Fe defects. Density functional theory based computed Se and Nb K-edge XANES are compared with their experimental counterparts. Theoretical spectra very well reproduce the observed trends of the experimental spectra. Our calculations in presence and absence of core-hole effect are presented separately, to understand the role of core-hole in determining XANES for these materials. In general, theoretically computed XANES spectra in the absence of core-hole effect agree significantly better with the experimentally observed one. All the absorption spectra consist of several peaked features of particular significance, which are thoroughly analyzed in terms of the unoccupied partial density of states. Presence of magnetic impurities like Fe, Cr cause inter-layer as well as intra-layer charge transfer. Bader charge analysis show that, more and more charges are gained by Se atom with increasing Cr defect concentration which shifts its absorption edge to lower photon energy. Whereas, loss of charge by Se atom with increasing Fe impurity shifts the corresponding absorption edge to higher photon energy. Nb atom gains charge in presence of Fe/Cr defects resulting its edge to shift to lower photon energy. These trends in absorption edge shift are similar in theoretical as well as in experimental spectra. We also provide theoretical prediction of Nb L 2 -edge which accesses Nb-d states that are relevant for superconductivity. [ABSTRACT FROM AUTHOR]