Some properties of the metallic state II—Metallic hydrogen and deuterium

The heat of formation of 1 g. atom of metal from the atoms has been calcu­lated to differ by about 1·6 kcal. for hydrogen and deuterium (Wigner and Huntingdon 1935). Allowing for the difference in the zero point energies of molecular hydrogen and deuterium (1·8 kcal. /mol. according to Rideal and Melville (1935)), the heat of formation of the metal from molecular hydrogen is smaller by 0·7 kcal./g. atom than from molecular deuterium. The difference in zero point energy between H and D in theα-phase is 0·5 kcal. /g. atom (Rideal and Melville). No data are available for theβ-phase. This difference is about 6 % of the heat of solution in forming theβ-phase, and since the expansion of the palladium lattice is sensitive to small energy changes (cf. Part I), an appreciable difference in expansions may be expected in the two alloys (cf. the discussion of other types of chemical linkage, Ubbelohde (1936). Measurements of the lattice expansion, and change of resistance on alloying with the two metals were undertaken to test this.