Snoek type relaxations and tunnelling of hydrogen

Snoek-type relaxations due to trapped hydrogen in crystalline and amorphous materials are reviewed and new results in crystalline alloys are presented. Close similarities are pointed out between anelastic processes observed in crystalline metals containing high concentration of hydrogen trapping centres, and those reported in metallic glasses. The H-relaxation processes in both types of materials can be interpreted by a unique statistical model by introducing a distribution of the energy for the interstitial sites whose occupancy by hydrogen is described by Fermi-Dirac statistics.