Ammonium ion dynamics in NH4I at high pressure

Measurements of spin-lattice relaxation time T-1 have been performed for po lycrystalline ammonium iodide NH4I as a function of hydrostatic pressure an d temperature. Activation parameters were obtained for different models of ammonium ion reorientations. It was found that the dominant mechanism for a mmonium reorientational motion in the ordered phases III and IV was the reo rientation around C-3 axes. Reorientation activation volume and libration f requency of ammonium ions as a function of pressure were calculated for dif ferent phases.