Why changes in bond lengths and cohesion lead to core-level shifts in metals, and consequences for the spatial difference method

Abstract

In metals, the chemical shifts of core-level energy loss spectra are largely determined, not by charge transfers, but instead by valence band shifts. The valence band shifts in turn are determined by changes in bandwidth, which result from changes in the type, number and distance to neighboring atoms. The core-level shifts tracks the valence-band shifts to within 0.1 eV, thus providing information on the occupied electronic states. As a consequence, core-level shifts are almost unavoidable at interfaces and cannot be ignored when analyzing data obtained by the `spatial’ difference method. Core-level shifts introduce first-derivative-like features in spatial difference spectra, that under typical conditions will be larger than the changes in energy-loss fine structure. Fortunately, it is far simpler to connect the core-level shifts to changes in cohesive energy than parameterizations of the fine structure, such as charge transfers. Reinterpreting spatial difference measurements of Cu : Bi, Fe : B and Fe : P grain boundaries as arising from core-level shifts may reconcile the experimental measurements with existing electronic structure calculations.