1H NMR study of natural and partially deuterated ammonium hexachlorotellurate.

The proton spin-lattice relaxation times in natural and partially deuterated ammonium hexachlorotellurate have been measured. At high temperature (above 50 K) the relaxation curve reveals the classical, hindered reorientation of the ammonium ion with the same activation energy for all studied samples Ea = 5.6 kJ/mol. Below 50 K, the minima which appear in the T1 temperature dependencies are related to the level-crossing. The tunnelling splitting of the ground torsional level in isotopic pure ammonium hexachlorotellurate is equal to 55 MHz and increases with increasing deuteron concentration, due to the phase transitions occurring in partially deuterated crystal. The lowest temperature minimum in isotopic pure (NH4)2TeCl6 can be interpreted as a result of the rigid ammonium ion's reorientation about the threefold crystallographic axis. The limited jumps of NH4+ are characterised by the angle delta = 4 degrees between the NH bond and [111] direction.