Dynamics of hydrogen in scandium and yttrium by acoustic spectroscopy

Acoustic spectroscopy has been employed to investigate the nature of the processes occurring at liquid He temperature in pure polycrystalline Y and Sc for frequencies in the kHz range. The anelastic spectra indicate that hydrogen is delocalized in at least two different tunnel systems. Possible tunnel systems are discussed: the isolated or interacting H atom tunnelling between pairs of adjacent tetrahedral sites, the H atom tunnelling near single or paired O traps. It is concluded that, in the present dilute alloys, the observed relaxations have to be attributed to tunnel systems constituted by H trapped by O displaying transition rates governed by the multiphonon process.