A kinetic study of the I to II phase transition in ammonium chloride, ammonium bromide and ammonium iodide

A kinetic study of phase transition between phases I and II in ammonium chloride and ammonium bromide has been carried out using time-resolved powder diffraction (TRPD) and differential scanning calorimetry (DSC). The same transformation in ammonium iodide has been studied using DSC only. Reaction onset temperatures for NH₄Cl, NH₄Br and NH₄I of about: 458 K, 412 K, and 253 K and enthalpies of transformation of: 4.4(2) kJ mol^− 1, 3.3(1) kJ mol^− 1 and 2.6(5) kJ mol^− 1 were determined. The Avrami equation adequately described the shape of the degree of reaction curve for all three materials. For NH₄Cl and NH₄Br, the Avrami shape factor was equal to two. Taken in conjunction with previous microscopic studies, this implies very rapid nucleation followed by two-dimensional growth of the new phase. The value of the shape factor for NH₄I switched from 2 to 4 about 55 s after the commencement of the transformation. Values of 50(20) kJ mol^− 1 and 20(10) kJ mol^− 1 were obtained for the activation energy before and after the switch. Activation energies for NH₄Cl and NH₄Br were found to be 400(80) kJ mol^− 1 and 170(80) kJ mol^− 1 from the DSC data. We conclude that for all three ammonium halides this transformation probably has the same mechanism.