Core electron spectroscopic studies on new structure type iron based superconductors CaKFe4As4 and KCa2Fe4As4F2: DFT predictions

Abstract

Theoretical predictions on X-ray absorption near edge structure (XANES) for two different newly discovered hybrid iron-based superconducting compounds, CaKFe4As4 and KCa2Fe4As4F2, are presented. The Fe and As K-edge absorption spectra of these hybrid compounds as well as their parent building block materials KFe2As2, CaFe2As2, and CaFeAsF are theoretically calculated and contrasted with each other. In particular, for the hybrid compounds, As K-edge absorption spectra are qualitatively different for two distinct types of As atoms belonging to dissimilar coordination environments. No such differences are found in the XANES of the building block compounds, in which all of the As atoms belong to an equivalent coordination environment. Our density functional theory-based first principles studies in the presence and absence of the core-hole effect are presented separately, to understand the role of the core-hole effect in determining the XANES for these compounds. In general, the core-hole effect is most significant in CaKFe4As4 compared to the other compounds, for which its effect is moderate. The distinct peak features of the absorption spectra signify specific electronic transitions, which are thoroughly explained in terms of the unoccupied atom projected partial density of states. Very prominent pre-edge features found in KFe2As2, CaFeAsF, and CaKFe4As4 imply perfect tetrahedral coordination of the absorbing Fe atom in these compounds, while this is slightly distorted in CaFe2As2 and KCa2Fe4As4F2. The interrelationship between the inter-atomic charge transfer and the shift in the Fe K-edge absorption for various compounds is established through Bader charge analysis. We find that the Fe atom transfers relatively less charge in the case of KFe2As2 compared to KCa2Fe4As4F2, which shifts the corresponding absorption edge of KCa2Fe4As4F2 to higher photon energy.