Systematics of the metallization of low-Z and alkali fluids

Abstract

Hydrogen, nitrogen, oxygen, cesium, and rubidium undergo nonmetal–metal (NMM) transitions in the degenerate warm fluid phase. It is quite likely that all these fluids are monatomic or very nearly so. For N, O, and H, these NMM transitions occur under quasi-isentropic compression to ∼100 GPa (1 Mbar) pressures and densities of ∼10 times initial liquid density in the case of H. These conditions were achieved with a two-stage gun. In the cases of Cs and Rb, these transitions occur at only ∼0.01 GPa in the expanded fluid at 2000 K. These NMM transitions are Mott transitions. The values of the minimum metallic conductivities are essentially the same for all five because minimum metallic conductivity depends weakly on density of metallization and number of conduction electrons per atom. In contrast, the density dependences of the semiconductivities are very different. In the spirit of Mott, quantum mechanical wave functions of the free atoms are used to estimate the densities at which semiconductivies are appreciable. The radial extents of the charge-density distributions are well correlated with the Mott-scaled density dependences of the semiconductivities. These radial extents depend on the degree to which the filled-electron core screens the valence electron(s) from the nuclear Coulomb force. This simple picture gives a qualitative explanation for the density dependences of the semiconductivities of all five and for the Herzfeld criterion, which predates quantum mechanics.