High pressure core electron spectroscopy in 111 Fe-based superconducting materials: A first principles study

Abstract

Based on particle swarm optimization algorithm, symmetry constraints on structural generation, crystal structures of 111 Fe-based superconducting materials at very high pressures were predicted by Zhang et al. (2012). Using these crystal structures we calculate Fe K-edge absorption spectra for NaFeAs and LiFeAs materials at very high pressures ranging from 22 GPa to 240 GPa using density functional theory. Theoretically computed Fe K-edge absorption spectra at different high pressures contain non-identical spectral features, which corresponds to different local structural environment around the absorbing atom due to applied pressure. At higher energies and higher pressures LiFeAs exhibits larger number of prominent features than that in NaFeAs. Origin of all the core electron absorption features is thoroughly analyzed in terms of the unoccupied partial density of states of the absorbing and ligand atoms. The role of core–hole effect on absorption spectra is also studied in detail. Bifurcation of spectral features in presence of core–hole effect and the corresponding transfer of spectral weights are also discussed in detail. At a moderate pressure 40 GPa, NaFeAs has higher Fe-K edge absorption energy (by ~3 eV) in comparison to that of LiFeAs; in contrast, LiFeAs shows larger inter layer charge transfer than NaFeAs. In general, the interrelationship between the interlayer charge transfer and the shift in the Fe K-edge absorption at various pressures is established through Bader charge analysis, where smaller the inter-layer charge transfer larger the shifts of the absorption edge to higher photon energies is found. In absence of any high pressure experimental X-ray absorption near edge structure (XANES) data, we show that our calculation agrees with available experimental data at atmospheric pressure reasonably well.