NMR studies of localized interstitial hydrogen motion in the h.c.p. metals Sc, Y and Lu

Abstract

The hexagonal close packed (h.c.p.) solid solution (α) phases of the Sc-H, Y-H and Lu-H systems have been investigated by nuclear magnetic resonance (NMR) methods over the temperature range 10–300 K in which hydrogen pairs are known to form. At low temperatures (10–120 K), localized motion of hydrogen between closely spaced (about 1.35 Å) tetrahedral (T) interstitial sites gives rise to a peak in the proton spin-lattice relaxation rate R 1 in α-ScH x and α-LuH x . This peak may be interpreted in terms of classical over-barrier hopping or tunnelling through the barrier between the close sites. The temperature and resonance frequency dependence of the R 1 peak in α-ScH x exhibit characteristics typically found in amorphous and disordered systems, and which have been interpreted in terms of two-level systems (TLS). This leads to the suggestion that pair formation results effectively in a “proton glass”. At a resonance frequency of 24 MHz, the proton R 1 maximum at about 40 K in α-ScH 0.11 corresponds to a hydrogen hopping frequency of v ∼- 1.5 × 10 8 s −1 . This value is approximately two orders of magnitude slower than the minimum rate (about 1.6 × 10 10 s −1 ) found at about 100 K in recent quasielastic neutron scattering (QENS) measurements on single-crystal α-ScH 0.16 . A hopping rate of the latter magnitude would not be detected by NMR measurements, so that the NMR and QENS measurements see different fast localized hydrogen motions. An interpretation based on recent theoretical work is presented.