Nuclear magnetic resonance evidence of disorder and motion in yttrium trideuteride

Three samples of YDx, with x ranging from 2.9 to nearly 3.0, were studied w ith deuterium nuclear magnetic resonance to gain insight into the locations of the D atoms in the lattice and their motions. Line shapes at low temper atures (200-330 K) show substantial disorder at some of the deuterium sites . Near 355 K, the spectrum sharpens to yield three uniaxial Fake patterns, reflecting a motional averaging process. However, the three measured intens ities do not match the ratios expected from the neutron-determined, HoD3-li ke structure. This is strong evidence that the structure and space group of YD3 are different than reported, or that the current model needs adjustmen t. At still higher temperatures near 400 K, the Fake doublet features broad en, and a single sharp resonance develops, signalling a diffusive motion th at carries all D atoms over all sites. The temperature at which line shape changes occur depends on the number of deuterium vacancies, 3-x. The change s occur at lower temperatures in the most defective sample, indicating the role of D-atom vacancies in the motional processes. The longitudinal relaxa tion rate T-1(-1) displays two regimes, being nearly temperature independen t below 300 K and strongly thermally activated above. The relaxation rate d epends on the number of deuterium vacancies, 3-x, varying an order of magni tude over the range of stoichiometries studied and suggesting that D-atom d iffusion is involved. Also, the activation energy describing T-1(-1) (simil ar or equal to k(B)x 5500 K) approximately matches that for diffusion. An u nusual omega(0)(-0.7) frequency dependence of T-1(-1) is observed. A relaxa tion mechanism is proposed in which diffusion is the Pate-determining step and in which frequency dependence arises from a field-dependent radius of t he relaxation zones. [S0163-1829(98)07845-X].