Ground state and excited state hydrogen tunnelling in Nb 1− xTi xH y

Abstract

Ti represents a substitutional dopant in Nb and at low temperatures acts as hydrogen trap. For Nb 0.999Ti 0.001H 0.001 at 1.5 K we observe a pronounced (ground state) tunnelling peak at 0.2 meV and a second weak peak at 0.4 meV in high resolution neutron spectra. The vibrationally excited state has been studied by neutron energy loss spectroscopy on Nb 0.99Ti 0.01H 0.0094; the (fundamental) vibrational peak around 110 meV exhibits a splitting into three tunnelling lines seperated by about 4 meV. Both tunnelling phenomena indicate hydrogen delocalization over four (or perhaps six) sites. This conclusion is supported by muon spin relaxation measurements between 10 and 200 K and a QENS study at 150 K, where in both cases a localized motion (jump rotation) of the light interstitials over four sites was found.