D2NMR andCl35nuclear-quadrupole-resonance study of ammonium-ion motion and phase transitions in natural and deuterated (NH4)2TeCl6

The ${}_{}{}^{35}{}_{}{}^{}\mathrm{Cl}$ nuclear quadrupole resonance (NQR) and the quadrupole perturbed ${}_{}{}^2{}_{}{}^{}\mathrm{D}$ NMR have been investigated in (${\mathrm{ND}}_4$)${\mathrm{TeCl}}_6$ in the temperature range 4.2–85 K. The ${}_{}{}^2{}_{}{}^{}\mathrm{D}$ 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.