1H NMR study of molecular dynamics and phase transition in (NH4)2ZnBr4

The proton second moment (M ₂) and spin-lattice relaxation time (T ₁) have been measured in (NH₄)₂ZnBr₄ in the range 77–300 K. The room-temperature spectrum shows a structure which disappears around 243 K. The signal is strong and narrow even at 77 K. Proton T ₁ shows a maximum at 263 K, caused by spin rotation interaction and decreases with decreasing temperature till 235 K, where it shows a sudden increase. Below 235 K, again it decreases and shows a slope change around 216.5 K (reported T_c ). From 216.5 K, T ₁ decreases continuously without exhibiting any minimum down to 77 K. The narrow line at 77 K, and absence of a T ₁ minimum down to 77 K indicate the possibility of quantum mechanical tunnelling in this system. Motional parameters such as activation energy and pre-exponential factor have been evaluated for the reorientational motion of the NH⁺ ₄ ion.