Deuteron spin-lattice relaxation in partially deuterated ammonium hexachlorotellurate

Deuteron spin-lattice relaxation is studied both theoretically and experimentally in polycrystalline samples of ammonium hexachlorotellurate with deuteron concentrations ranging from 5% to 100%. A model is derived for the initial deuteron relaxation rate, via the electric quadrupole interaction, under the assumption that the static part of the rotational Hamiltonian and ammonium rotation obeys the same symmetry. The obtained result reveals a possibility for maxima in the relaxation rate at level crossings, where the deuteron resonance frequency equals g|hi′-hj′| with g = 1/2, 1 or 2. The tunelling matrix element hj′ produces the tunnel splitting 3|hj′| for the three NH3D+ protons when the deuteron is at the equilibrium position j. The relaxation rates via the deuteron-proton magnetic dipolar interaction is considered. Experimental results are explained by the above model based on the quadrupole interaction at higher deuterations and by so-called limited jumps at lower deuterations. Limited jumps mean a restricted rotation of the rigid ammonium ion in such a way that the nitrogen-hydrogen bonds make always a certain angle with the nearest nitrogen-tellurium direction. Our interpretation of relaxation rates agrees with the details of deuteron NMR spectra.