Recent work with metal-hydrogen systems (those in which the metal is a transition metal)
has given considerable support to the idea, advanced by Hill and by Alefeld, that such metal-
hydrogen systems are excellent examples of a lattice gas system—something more usually
regarded in physics as a theoretical model, as developed by Yang and Lee. These
metalhydrogen systems do not behave as simple lattice gases, because the metal atoms
providing the lattice gas cells are influenced by the actions of the interstitial hydrogen so that …