Tunnelling of H trapped by substitutional Zr in Nb: an investigation on its geometry

We carried out an investigation on the symmetry of the tunnel system of H trapped by Zr in b.c.c. Nb. We measured the elastic energy dissipation of two NbZr0.0045Hx single crystals excited with extensional and torsional vibrations along their [100] and [111] directions at frequencies around 40 kHz. Below 40 K at least two processes are observed under ?110? shear (c11-c12); under other types of stress the intensity of the main contribution is reduced by about a factor of 5, while a smaller peak around 2 K is completely suppressed. Tunnelling over two or four sites in the perturbed region close to the substitutional impurity is considered; the absence or strong reduction of relaxation under ?100? shear can be explained by the symmetry of a four-site tunnel system.