Deuterium NMR study of structure and motion in LuD₃

Deuterium NMR spectra of LuD₃ were obtained at 8.0 and 8.4 T from 163 to 398 K, revealing only minor changes with temperature. The resonance has an outer, uniaxially symmetric quadrupolar doublet, arising from D_m atoms in the unit cell. An inner resonance with quadrupolar asymmetry η=0.68 and approximately twice the intensity is assigned to the nominally tetrahedral D_t atoms. Using partial alignment of the powder particles in the magnetic field and subsequent freezing in wax, the NMR line shape has been studied as a function of field direction, allowing the orientation of the η≠0 electric-field-gradient tensor to be partly determined. In addition, the agreement of the aligned line shapes with simulations confirms that the spectrum has only two components, from D_m and D_t atoms; specifically, there is no evidence for two subgroups of D_t atoms. The decays of two-pulse echoes and three-pulse spin-alignment echoes indicate fluctuations in the quadrupole splittings due to atomic motions. The spin-lattice relaxation rate R₁ above 300 K is an approximately thermally activated function of temperature; in this region R₁ is determined by atomic diffusion to relaxation centers. Comparisons are made between the ²D NMR behaviors of LuD₃ and YD₃.