D-2 NMR AND CL-35 NUCLEAR-QUADRUPOLE-RESONANCE STUDY OF AMMONIUM-ION MOTION AND PHASE-TRANSITIONS IN NATURAL AND DEUTERATED (NH4)2TECL6

The Cl35 nuclear quadrupole resonance (NQR) and the quadrupole perturbed D2 NMR have been investigated in (ND4)TeCl6 in the temperature range 4.285 K. The D2 spin-lattice relaxation rate shows a classical maximum at about 80 K and a broad peak at about 25 K, which is attributed to the influence of a phase transition at the same temperature. An extra contribution to the chlorine NQR spin-lattice relaxation rate is observed near the cubic-to-trigonal transition at 85 K. The disappearance of the NQR signal before the second transition at about 42 K is accompanied by a divergence of the spin-spin relaxation rate. The results are discussed in terms of a localization of the deuterium in a triangle-shaped potential well in the plane perpendicular to the N-D bond direction. © 1990 The American Physical Society.