Ab initio simulation of the electron energy-loss near-edge structures at the Li K edge in Li, Li2O, and LiMn2O4

Ab initio simulations of the electron energy-loss near edge structures (ELNES) at the Li K edge were obtained with the WIEN2k code, which is based on the density functional theory (DFT). Since the 1s states of the lithium atoms must be considered as semicore states, the standard procedure to calculate the dynamic form factor (DFF) has to be modified. The difficulties raised by the delocalized nature of these states and their influence on the simulated spectra are discussed. Unphysical monopolar transitions are shown to be extremely large and the description of the Li 1s orbital as a core or valence state for the self-consistent determination of potentials is proven to have little influence on the calculated spectrum shape. Comparisons between simulated and experimental spectra for three different compounds are presented: Li, Li2O, and LiMn2O4 (a metal, an insulator, and a transition metal oxide). All simulations fit well the experiments, thus demonstrating that this approach can be applied to a wide range of lithiated compounds. In the case of the Li K edge in Li2O, we also show that the recently proposed partition of the spectrum into two energy range regions, with different core hole strengths, improves the concordance between the simulation and the experiment. Finally, the influence of the real part of the dielectric function, omitted when using the DFF, is assessed. A comparison of Li K edges calculated using either the loss function [Im(-1/ε)] or only the DFF (imaginary part of ε) is presented in detail. We show that polarization effects are particularly strong in the Li K edge of LiMn2O4 due to the presence of the Mn M2,3 edge. Other calculations including the GW approximation and the local field effects would, however, be necessary to precisely quantify these effects