Phase transitions of hydrogen in metals due to elastic interaction

Abstract Hydrogen in metals can be considered as an example for a lattice gas with phase transitions. In the first part a phenomenological discussion of the phase diagrams of hydrogen in Pd, Nb, Ta and V will be given. It will be shown that these phase diagrams show topologically great similarity to those which hold for the phase transitions gas‐liquid‐solid of a one component system. In the second part the elastic dipole dipole interaction will be discussed. This interaction is considered to be the predominant part of the attractive interaction responsible for the “gas‐liquid” transition. Three experimental methods to measure the mean interaction energy of the lattice gas will be discussed. Experimental results for the mean interaction energy will be compared with theoretical predictions. Arguments are presented that the critical temperature for the gas liquid transition should be strongly decreased by embedding the sample into material with elastic coefficients stiffer than those of the sample.