H and D tunneling systems in diluted Nb1-xZrx alloys

The anelastic spectra of a single crystal of Nb0.9955Zr0.0045 containing controlled amounts of O and D have been measured to clarify the nature of a narrow peak due to H tunnelling, which appears only at the lowest impurity contents in the dilute Nb substitutional alloys. It is shown that the process is due to Zr-D pairs without the involvement of unwanted O impurities. The saturation of the peak intensity can be understood in terms of the increasing interactions among the H atoms and the consequent broadening of the distribution of the energy asymmetries of the tunnel systems.