Electronic structure of matter at high compression: Isostructural transitions and approach of the Fermi-gas limit

This paper is concerned with the effect of atomic-shell structure on the cold pressure of highly compressed matter up to 1 Gbar and above. The method is to compare band-structure calculations based on augmented spherical waves with results of the quantum-statistical model. Elements treated explicitly are Li, He, Be, Al, and K. No indication for anomalies due to pressure ionization is found. However, an oscillatory behavior of cold-pressure curves is systematically obtained due to isostructural electronic transitions. They are traced back to pressure-induced band shifts and electron redistribution from bands of low angular momentum (typically s and p waves) to bands of higher angular momentum (typically d and f waves). The shifts are shown to occur generally when squeezing normal matter with extended atoms into highly degenerate matter. The anomalies are particularly pronounced for alkali metals: Strong pressure flattening is found for Li at fivefold compression and 5–10 Mbar, and very sharply for K somewhat below fivefold compression and 500 kbar. The oscillations are much weaker for Be and Al. Metallization of He is obtained at 12.1 g/${\mathrm{cm}}^3$ and 110 Mbar. For Be, a structural hcp→bcc transition at 2 Mbar is predicted.