Lattice distortion, elastic interaction, and phase transitions of hydrogen in metals

Hydrogen dissolves in many metals and occupies interstitial sites in the host lattice. Lattice distortions expand the metal lattice and give rise to an elastic interaction between hydrogen. This elastic interaction is the relevant long range interaction for the phase transition α- α' of hydrogen in niobium. The α and α' phase show a close analogy to the gas and liquid phase of a real fluid. There is, however, an important difference which is also due to the elastic interaction. The coherent phase transition α- α' is accompanied by large coherency stresses which give rise to macroscopic and microscopic density modes. X-ray and neutron scattering methods were used to get information on lattice distortions, elastic interaction, and the hydrogen density modes. After an introduction to the scattering methods and outline of the physical concepts for phase transitions of hydrogen in metals recent experimental results are presented. (orig.)