Thermodynamically metastable plastic phase revealed in methylammonium bromide crystals

In the three solid phases I, II and III of methylammonium bromide obtainable above room temperature, the highest-temperature phase I (483–510 K), which is an ionic plastic phase, and also the high-temperature range (ca. 457–483 K) of phase II, are shown to transform spontaneously into liquid when maintained at any temperature in these temperature regions. The time required to complete the transformation increases with decreasing temperature reaching ca. 1500 h at 457 K. The free-energy curves in these phases depicted according to these results imply that phase I and at least the high-temperature range of phase II are in metastable states and that the stable phase in these temperature ranges is liquid.Microscopic molecular motions in phase II were studied by 1H NMR and electrical conductivity measurements revealing that rapid translational self-diffusion of the ions takes place in the high-temperature region. This motion can be considered to function as the trigger for the gradual melting of this phase.