Some properties of the metallic state I—Metallic hydrogen and its alloys

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Abstract
An investigation of the properties of metallic hydrogen is of particular interest, on account of the simple structure of a metal composed of protons and electrons. A calculation of the energy change on forming a body centred lattice of metallic hydrogen from hydrogen atoms has recently been made by Wigner and Huntingdon (1935). They find that the energy of formation of metallic hydrogen from hydrogen atoms would be 10 kcal., and of metallic deuterium from deuterium atoms 11·6 kcal., but that metallic hydrogen (Graham’s hydrogenium) would only be stable relatively to covalent hydrogen at pressures not less than 2·5 × 10 5 atmospheres. Experiments on hydrogen at these pressures have not yet been realized. It is however possible to study the properties of metallic hydrogen in a number of alloys. At high temperatures, hydrogen dissolves to an appreci­able extent in a number of metals (Sieverts and Gotta 1928), but at low temperatures the only examples where hydrogen dissolves appreciably without forming covalent links (as GeH 4 ) or negative ions (as LiH) are the “metallic” hydrides of transitional elements such as palladium, tantalum, titanium, etc. The exceptional position of these low temperature alloys of metallic hydrogen is referred to again below. The evidence that hydrogen is in the metallic state has been reviewed (Ubbelohde 1931) and may be summarized by the statements that the hydrogen dissolves as atoms, and that at least a portion of these atoms is ionized to give electrons and protons.