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.