Electronic and x-ray-absorption structure in compressed copper

Abstract

Using electronic band-structure techniques, we calculate the K- and L-edge absorption spectra for fcc copper over a wide range of compressions. As copper is compressed the x-ray-absorption structure greatly increases and copper deviates more and more from its normal-density free-electron-like behavior. We explain this additional structure in terms of an increased scattering amplitude with pressure and relate it to both extended x-ray-absorption fine-structure (EXAFS) theory and a simple Friedel-model calculation. We show that specific band symmetry points can be well correlated with the x-ray-absorption near-edge structure features, but that this procedure is useless at higher energies above the edge (the EXAFS region). We suggest that the energy dependence of the peaks in the spectra can be used as a high-pressure volume diagnostic. We also discuss general complications and limitations in our knowledge about high-pressure x-ray-absorption spectra as well as overall changes in the electronic structure of copper under compression. To correlate our results with applied pressure, we also present zero-temperature equation-of-state calculations (pressure versus volume).