Anomalous hydrogen dynamics in rare earth metals

Abstract

Neutron scattering studies of the dynamics of the hydrogen isotopes in h.c.p. rare earth metals have demonstrated a rich variety of phenomena of basic importance to the understanding of hydrogen in metals. In this paper we review our recent measurements of hydrogen vibrations in yttrium using inelastic neutron scattering and our very recent investigations of hydrogen local hopping in scandium and yttrium using quasielastic neutron scattering. Using simple models we attempt to provide a straightforward but fairly comprehensive picture of the way extended hydrogen short range order, observed directly in diffuse neutron scattering experiments, influences these measurements. We show, for example, how the vibrational peak splitting measured in α-YHx can be interpreted in terms of temperature- and concentration-dependent effects of the chain-like pairing order. We also describe in detail our recent demonstration of very fast local T-T hopping in α-ScHx and in α-YHx and relate the observations to the identification of thermodynamically distinct labile (uninvolved in extended order) and non-labile (involved in extended order) hydrogens. In particular, we measure a pronounced T-T hopping rate minimum at 100 K, which we ascribe to the labile protons and show how the low temperature hopping rate is described by the Kondo model of quantum hopping due to the non-adiabatic effects of weak interaction between the protons and the conduction electrons of the host metal.