1H NMR and Thermal Studies of CH3NH3Br in a Metastable Solid Phase Newly Found above 483 K

By differential thermal analysis a new high-temperature solid phase of methylammonium bromide was found between 483 K and its “melting point” (510 K). 1H NMR absorption measurements revealed the presence of rapid 3D translational self-diffusion and overall rotation of methylammonium cations in this phase. These cationic motions are quite analogous to those of methylammonium iodide in its ionic plastic phase. Surprisingly, this plastic-like phase is metastable, the stable phase in the same temperature range being liquid.