Pressure ionization and electron localization in solid silicon and cobalt under extreme pressure*

Under extreme pressure, plasmas undergo pressure ionization, in which the atomic potential depth is reduced, and continuum lowering, in which the X-ray absorption onset shifts towards lower energy. We previously found by density functional theory (DFT) calculations that solid silicon shows pressure ionization but not continuum lowering because of Fermi-surface rising as in dense plasmas. Now we further investigate solids at low temperature but extreme pressure (~100 Mbar). Under these conditions, simple metals such as sodium become electrides (ionic-like insulators), and we found signatures of electride formation in diamond-structure silicon as well. The potential depth can be fit by common ionization models (ion-sphere, Debye-Hückel, modified Ecker-Kroll, and Stewart-Pyatt). We improve on our previous DFT and random phase approximation calculations with the GW/Bethe-Salpeter approach for more accurate electronic phase transitions and X-ray spectra. We also examine solid cobalt to see the effect of d orbitals in a transition metal, calculating potential depth, bandstructure, and X-ray spectroscopy, and look for signs of electride formation. This investigation gives insight into the connection between solid and plasma phenomena in extreme compression experiments.