Hydrogen—The Lightest Alkali Metal

Weir et al. (1996) measured the electrical conductivity of their hydrogen sample in situ using electrodes inserted through the walls of the cell, flush with the fluid hydrogen-sapphire interface. The conductivity rises continuously over five orders of magnitude with pressure increasing from 

4 Mbar to 1.8 Mbar. When we compare the conductivity versus density curves of hydrogen with those obtained for rubidium and cesium at comparable temperatures in Fig. 3.22, it is clear that all three elements undergo a density-induced transition to a highly conducting state (o-(O) 2000 fl cm ). But the density at which this occurs for fluid hydrogen is about 100 times that required for rubidium and cesium. 

It is reasonable to speculate that the differences in elemental densities at the MNM transition are related to characteristic atomic properties. One such property, for example, is the radius of the principal maximum in the charge density of the ns valence orbital, a which enters into the Mott criterion (Section 2.3.4). A related property is the static polarizability a of the isolated atom. The polarizability formed the basis of very early discussions of the MNM transition by Goldhammer (1913) and Herzfeld (1927). They pointed out that electrons localized around atomic nuclei constitute polarizable objects and their internal dynamics in dense assemblies leads to local corrections to the polarizing tendency of any external field impressed on the system. For an isotropic material, the correction factor has the form [1 — (4Tr/3)lVa] where N is the number of atoms per unit volume. If a is taken to remain roughly 

Abeles, F. 1972. Optical Properties of Solids. Amsterdam North-HoUand. Abragam, A. 1961. Nuclear Magnetism. London Oxford. 

Adamson, A. W. 1976. Physical Chemistry of Surfaces. New York Wiley. 

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