NH3D+ dominated proton spin-lattice relaxation in partly deuterated ammonium compounds

Proton spin-lattice relaxation is studied in partly deuterated ammonium compounds at low temperatures. A model is proposed for the NH3D+ related contribution, which increases the relaxation rate many times larger than in nondeuterated samples. The model introduces two kinds of level-crossing minima in T1, where some tunnel frequency is equal to a separation between the proton Zeeman levels in the external magnetic field. One kind of minimum involves a CH3-type tunnel splitting of NH3D+, when the deuteron is stationary at anyone of the four threefold axes of the ammonium ion. The corresponding relaxation rate is expected to depend on the experimental pulse sequence. The other kind of minima (there could be six of them) result from the NH3D+ rotations moving the deuteron from one threefold axis to another and back, which make the otherwise motionally independent AA part of the magnetic dipolar interaction of NH3D+ time dependent. The involved tunnel splitting is equal to (2/3) times the difference between the CH3-type tunnel splittings. In a level-crossing transition the tunnel energy is changed by that splitting, but the resulting energy imbalance is transferred fast to the lattice by spin-state preserving reverse deuteron jumps, removing any coupling to the tunnel energy reservoir. Thereafter another level-crossing transition is possible. Experiments on polycrystalline 4% and 10% deuterated ammonium hexachlorotellurate samples reveal two additional level-crossing minima in T1 below 20K at the proton resonance frequencies 25MHz and about 28MHz, which are not found in the nondeuterated sample. The minima show different characteristics relative to the experimental pulse sequence and also agree otherwise well with the predictions of the model.