Electronic phase transitions in solids under extreme compression studied by hybrid DFT, DFT+U and GW approaches*

Under extreme pressure, solids can undergo electronic phase changes (such as semiconductor → metal, metal → electride, or ferromagnetic → paramagnetic), and these can happen even if the crystal structure is kept fixed. Our prior investigations using density functional theory (DFT) showed semiconductor → semimetal → metal → semimetal transitions in diamond structure silicon, and a metal → insulator transition in hcp cobalt, due to electride-like behavior. To improve on these DFT-PBE calculations, we use the hybrid HSE and DFT+U approaches, along with the beyond-DFT GW approximation, to identify more accurately the critical pressures for phase transitions of silicon and cobalt.  We study the charge density, density of states, electronic bandstructure, dielectric function, and magnetization of compressed silicon and cobalt. We also use Mermin-DFT to analyze the impact of electronic temperature. Furthermore, we assess the applicability of plasma-based ionization models (ion-sphere, Debye-Hückel, modified Ecker-Kröll, and Stewart-Pyatt) for solid cobalt and silicon. This investigation gives better insight into the generality of electronic phase changes in solids under extreme compression, and the degree to which they can be predicted by common ionization models.