Low-temperature localized motion of hydrogen and electronic structure transition in hexagonal-close-packed scandium

We report nuclear magnetic resonance (NMR) measurements of the proton (¹H) spin-lattice relaxation rate (R₁) in the hexagonal-close-packed (hcp) solid solution ({α}) phase of the Sc-H system over the temperature range 10--300 K in which hydrogen pairs are known to form. At low temperatures (10--120 K), fast localized motion of hydrogen between closely spaced tetrahedral interstitial sites in the lattice gives rise to a peak in the relaxation rate. Both the temperature and frequency dependences of the relaxation rate peak exhibit characteristics typical of amorphous and disordered systems, suggesting the formation of hydrogen pairs with little long-range order results effectively in a ``proton glass'' within the metal lattice. The measurements reveal an electronic structure transition near 170 K where the unpaired electron spin density at the proton sites decreases with increasing temperature.