Disorder and polymorphism of tetramethylammonium chloride

The tetragonal (III)/rhombohedral (II) and rhombohedral/f.c.c. (I) transition lines of tetramethylammonium chloride were measured as functions of pressure. The {III}/{II} transition is reversible in the presence of trace amounts of solvents. Phase II has a configurational entropy of ∼Rln {3}/{2}, and phase I a configurational entropy of ∼ Rln 32. It is suggested that this is due to orientational disorder of the cations, and that no major changes in proton disorder at the transitions are involved. A structure for tetramethylammonium chloride II, based on the space group R3 m, is proposed.