The Important Role of N(2)(CH3)4 Ion in the Phase-Transition Mechanism of [N(CH3)4]2ZnBr4

The chemical shift of the N(2)(CH₃)₄ ion, which has been found to exhibit the similar anomalous behavior of the monoclinic angle Aβ, was related to the order parameter to evaluate the temperature dependence of the linewidth (damping constant) for ¹⁴N nuclear magnetic resonance spectrum of this crystal in terms of the dynamic Ising models, namely the pseudospin-phonon-coupled (PS) and the energy fluctuation (EF) models. The results from both PS and EF models were successful to explain the abnormal behavior of the linewidth in the vicinity of the phase-transition temperature of T_C = 287.6 K, when compared with the observed linewidth of the transverse acoustic soft mode in this crystal. As an extension of this work, the ¹⁴N relaxation time and the values of the activation free energy were calculated as a function of temperature. The results indicate that the ferroelastic-paraelastic phase transition in this compound is of the order-disorder type.