Hydrogen four-level tunnel systems in substitutional body-centred cubic alloys

A brief account is provided of the main results of a study of the tunneling states of H trapped by substitutional (S) impurities in Nb, mainly consisting in anelasticity experiments. The phenomenology is rather complex when the concentration of S atoms is higher than a few parts per thousand, and various and contrasting interpretations are possible. The complication arises from the destruction of the symmetry of the S-H pair by the elastic interactions among the defects. The situation becomes, however, clear for an S content around 0.1%, when the anelastic spectra reveal the relaxation processes due to the now nearly undistorted S-H complexes. In this case H delocalizes over four tetrahedral sites of a face of the cube containing the S atom, giving rise to a four-level tunnel system (FLS). The parameters of tunneling and coupling to phonons and electron excitations are similar to those found for the two-level system of H near an interstitial impurity, but new effects are found, due to the symmetry of the additional eigenstates of a centrosymmetric FLS.