Stability of the Electronic Configuration and Compressibility of Electron Orbitals in Metals under Shock-Wave Compression

Abstract

Atomic radii in metals at 0°K are calculated from shock-wave equation-of-state measurements, and are compared with the radii of various free-atom electron orbitals obtained from Hartree-Fock calculations. For metals from the long periods of the periodic table having less than half-filled conduction bands, the $Z$ dependence of the experimental atomic radii and of the Hartree-Fock, free-atom orbital radii are found to be essentially identical at all pressures. This allows the identification of the dominant contribution to the effective interatomic interaction. In these metals it is found that the presence of a significant population in the $d$ band appears to result in a low compressibility. An unusually high compressibility observed for the normally trivalent rare-earth metals is then taken as evidence of the promotion of a $5d$ electron to a $4f$ shell under compression. Interactions between closed electron shells in metals are estimated from the experimental equations of state of the rare gases and their isoelectronic alkali halides. In the experimental pressure range, interactions between these closed-shell cores are found to be important only for the rare-earth metals, where an observed stiffening of the Hugoniot is identified as resulting from core interactions. The limits of validity of Thomas-Fermi—like equation-of-state calculations are discussed.