High temperature phase transitions in (CH3)4NCl

Abstract

The high temperature structural modifications of tetramethylammonium chloride [(CH3)4NCl] which exist between 298 and 607°K were investigated by static and rotating frame proton spin-lattice relaxation times T1 and T1p, respectively. Polycrystalline samples treated in different manners were studied in order to understand the mechanism of the phase transition near 418°K. Two possible processes are consistent with our observations. At the transition temperature to the high temperature cubic form, 536°K, a discontinuity in the spin-lattice relaxation times was evident. We find that T1 either depends on temperature very little, or is temperature invariant in the cubic phase, while T1ρ reveals the onset of a very slow (τC>10−4 sec) self-diffusion of the cations with an activation energy of about 20± 3 kcal/mole. The diffusion may be associated with the decomposition of the salt.