Hopping and tunnelling of D trapped by substitutional Zr in Nb single crystals

The hopping and tunnelling of deuterium trapped by a Zr substitutional atom in Nb have been investigated by elastic energy loss measurements. A Nb0.995Zr0.0045Dx single crystal was excited in its c? and c44 modes at frequencies between 6.3 and 130 kHz, from 1 to 300 K. The results are compared with previous experiments on the same sample doped with H and on Nb1-yTiy(H,D)x. The main peak around 150 K due to hopping of D around Zr is accompanied by secondary peaks at lower temperature, like in the alloys previously investigated. This indicates that D (as well as H) has two distinct types of occupation around the Zr atom. The most striking feature is the isotope dependence of a narrow peak at 2 K due to H (D) tunnelling. The intensity of the peak with D is more than one order of magnitude higher than that with H. An explanation of this fact is proposed, which only requires the reduction of the tunnelling matrix element when passing from H to D. Tunnelling within both two or four equivalent sites near the Zr atom is considered.