Anelastic relaxation of H trapped by Zr in Nb single crystals

The anelastic relaxation due to the reorientation of the ZrH pair has been measured in two Nb single crystals at low H and Zr contents. The two crystals were oriented with the longer dimension along the [100] and [111] directions, and were excited in their extensional, torsional and flexural modes. The observed dissipation between 50 and 200 K consists of at least two peaks. One of them is not observed under [100] torsional and [111] extensional vibrations, and this is interpreted as due to the redistribution of H among four-site tunnel systems over the faces of the cube containing the Zr atom. The other peak is predominant and is always observed, implying the occupation of additional sites.