Deuterium NMR study of structure and motion in (formula presented)

Deuterium NMR spectra of (formula presented) 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 (formula presented) atoms in the unit cell. An inner resonance with quadrupolar asymmetry (formula presented) and approximately twice the intensity is assigned to the nominally tetrahedral (formula presented) 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 (formula presented) 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 (formula presented) and (formula presented) atoms; specifically, there is no evidence for two subgroups of (formula presented) 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 (formula presented) above 300 K is an approximately thermally activated function of temperature; in this region (formula presented) is determined by atomic diffusion to relaxation centers. Comparisons are made between the (formula presented) NMR behaviors of (formula presented) and (formula presented).