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

Abstract

The proton second moment ( M 2 ) and spin-lattice relaxation time (TI) have been measured in (NH,),ZnBr, 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 I 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,). From 216.5 K, TI decreases continuously without exhibiting any minimum down to 77 K. The narrow line at 77 K, and absence of a T, 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 NHL ion.

Role of NH4 ions in successive phase transitions of perovskite type (NH4)2ZnX4 (X = Cl, Br) by 1H MAS NMR and 14N NMR

“…22 The low-temperature phase IV is orthorhombic with a ¼ a o , b ¼ b o , c ¼ 3c o , and space group P2 1 cn. 22 The 1 H spin-lattice relaxation times of (NH 4 ) 2 ZnCl 4 and (NH 4 ) 2 ZnBr 4 crystals have been obtained in the laboratory frame by Lim et al [23][24][25] and Ramesh et al, 26 respectively. The molecular dynamics and phase transitions of (NH 4 ) 2 ZnCl 4 single crystals were reported previously.…”