Nuclear magnetic resonance evidence of disorder and motion in yttrium trideuteride

Abstract

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, $3\ensuremath{-}x.$ 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 ${T}_{1}^{\ensuremath{-}1}$ 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, $3\ensuremath{-}x,$ varying an order of magnitude over the range of stoichiometries studied and suggesting that D-atom diffusion is involved. Also, the activation energy describing ${T}_{1}^{\ensuremath{-}1} (\ensuremath{\simeq}{k}_{B}\ifmmode\times\else\texttimes\fi{}5500 \mathrm{K})$ approximately matches that for diffusion. An unusual ${\ensuremath{\omega}}_{0}^{\ensuremath{-}0.7}$ frequency dependence of ${T}_{1}^{\ensuremath{-}1}$ 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.