A calorimetric contribution to the study of polymorphism in tetramethylammonium chloride

The heat capacity of tetramethylammonium chloride has been measured from 115 to 498 K, both for the thermodynamically stable modifications and for the form II, which is metastable below 407 K. The entropy gains at the IV → III λ-transition and at the III → II first-order transition are estimated to be 1.19 J K⁻¹ mol⁻¹ and 4.16 J K⁻¹ mol⁻¹, respectively. At 125 K, the entropy of the metastable phase II exceeds that of the stable modification IV by only 1.76 J K⁻¹ mol⁻¹. If, as has been suggested, each cation in phase II has two possible orientations, it would seem that at 125 K this disorder in phase II is largely, if not wholly, suppressed, without the appearance of any transition or thermal anomaly.Results are also recorded of test measurements of the heat capacity of the NBS sample of synthetic sapphire (α-Al₂O₃) from 138 to 515 K.