Study of internal motions and phase transitions in (NH4)2ZnCl4 and [(N(CH3)4)]2 ZnCl4 by proton magnetic resonance and relaxation

Abstract

Proton second moment (M2) and spin-lattice relaxation time (T1 at 10 MHz have been measured in polycrystalline ammonium tetrachlorozincate (ATC-Zn) and tetramethyl ammonium tetrachlorozincate (TMATC-Zn) as a function of temperature, covering the phase transitions. In ATC-Zn, the motion of NH+4 ion is fast in all phases as seen by a small and temperature independent proton second moment. Two types of chemically inequivalent NH+4 ions have been identified from the observed double minima in proton T1. Proton T1 shows a discontinuous jump and change of slope at 319 K, probably due to a phase transition. In addition, T1 shows a maximum around the reported ferroelectric phase transition (270 K), indicating the presence of spin-rotation interaction. In TMATC-Zn, the lowest temperature transition seems to be related to the slowing down of CH3 groups as revealed by the proton second moment. In this system also, two kinds of [N(CH3)4] ions have been found to be present from the proton T1 studies. Proton T1 shows change of slope at the incommensurate to prototype transition. From the experimental data the activation energies have been evaluated and the role of the cation at the observed phase transitions in both the compounds is discussed.