Nuclear magnetic resonance study of the low-temperature localized H(D) motion in alpha-ScHx(D-x): Isotope effects

Nuclear magnetic resonance measurements of the (SC)-S-45, H-1 and D-2 spin- lattice relaxation rates in the solid solutions alpha-ScHx (x=0, 0.11, and 0.27) and alpha-ScDx (x=0.05 and 0.22) have been performed over the tempera ture range 4.2-294 K. For the hydrided and deuterided samples we have found pronounced deviations of the (SC)-S-45 relaxation rate measured at 86.2 MH z from the Korringa behavior in the range 30-180 K. The excess (SC)-S-45 re laxation rate displays a peak near 100 K in alpha-ScHx and near 110-140 K i n alpha-ScDx, resulting from the quadrupole interaction modulated by fast l ocalized hopping of H(D) atoms. The amplitude of the (SC)-S-45 relaxation r ate measured at 86.2 MHz from the Korringa behavior in the range 30-180 K. The excess (SC)-S-45 relaxation rate peak in the deuterided samples is foun d to be much higher than in the hydrided samples with comparable hydrogen c ontent. This unusual isotope effect indicates that the fraction of D atoms participating in the fast localized motion in alpha-ScDx is approximately s ix times as large as the corresponding fraction of H atoms in alpha-ScHx. T he results of our measurements of the H-1 and D-2 relaxation rates are cons istent with this conclusion. The strong effect of H<-> D substitution on th e fraction of atoms involved in the localized hopping should be accounted f or in any microscopic model of hydrogen motion in Sc. [S0163-1829(99)15725- 4].