H-1-NMR STUDY OF NATURAL AND PARTIALLY DEUTERATED AMMONIUM HEXACHLOROTELLURATE

The proton spin-lattice relaxation times in natural and partially deut erated ammonium hexachlorotellurate have been measured. At high temper ature (above 50 K) the relaxation curve reveals the classical, hindere d reorientation of the ammonium ion with the same activation energy fo r all studied samples E-a = 5.6 kJ/mol. Below 50 K, the minima which a ppear in the T-1 temperature dependencies are related to the level-cro ssing. The tunnelling splitting of the ground torsional level in isoto pic 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 mini mum in isotopic pure (NH4)(2)TeCl6 can be interpreted as a result of t he rigid ammonium ion's reorientation about the threefold crystallogra phic axis. The limited jumps of NH4+ are characterised by the angle De lta = 4 degrees between the NH bond and [111] direction. (C) 1998 Else vier Science B.V. All rights reserved.