Nuclear magnetic resonance evidence of disorder and motion in yttrium trideuteride

Three samples of ${\mathrm{YD}}_x,$ with x ranging from 2.9 to nearly 3.0, were studied with deuterium nuclear magnetic resonance to gain insight into the locations of the D atoms in the lattice and their motions. Line shapes at low temperatures (200–330 K) show substantial disorder at some of the deuterium sites. Near 355 K, the spectrum sharpens to yield three uniaxial Pake patterns, reflecting a motional averaging process. However, the three measured intensities do not match the ratios expected from the neutron-determined, ${\mathrm{HoD}}_3$-like structure. This is strong evidence that the structure and space group of ${\mathrm{YD}}_3$ are different than reported, or that the current model needs adjustment. At still higher temperatures near 400 K, the Pake doublet features broaden, and a single sharp resonance develops, signalling a diffusive motion that carries all D atoms over all sites. The temperature at which line shape changes occur depends on the number of deuterium vacancies, $3-x.\mathrm{}$ The changes occur at lower temperatures in the most defective sample, indicating the role of D-atom vacancies in the motional processes. The longitudinal relaxation rate $T_1^{-1}$ displays two regimes, being nearly temperature independent below 300 K and strongly thermally activated above. The relaxation rate depends on the number of deuterium vacancies, $3-x,$ varying an order of magnitude over the range of stoichiometries studied and suggesting that D-atom diffusion is involved. Also, the activation energy describing $T_1^{-1}\hspace{0.5em}(\simeq k_B\times 5500\hspace{0.5em}\mathrm{K})$ approximately matches that for diffusion. An unusual ${\omega }_0^{-0.7}$ frequency dependence of $T_1^{-1}$ is observed. A relaxation mechanism is proposed in which diffusion is the rate-determining step and in which frequency dependence arises from a field-dependent radius of the relaxation zones.