of hydrogen in metals. The crystal lattice can expand with the concentration of impurity interstitials The amorphization of semiconductors such as silicon during ion irradiation is often explained by the build up of a high concentration of interstitials leading eventually to the collapse of the lattice as it becomes unstable. Creation of large amounts of interstitials in a solid can lead to a significant energy buildup, which on release can even lead to severe accidents in certain old types of nuclear reactors (Wigner effect). The high-energy states can be released by annealing. At least in fcc lattice, interstitials have a large diaelastic softening effect on the material. It has been proposed that interstitials are related to the onset of melting and the glass transition. References Crystallographic defects Mineralogy concepts Materials science Alloys Metallurgy