Structural investigations of the phase transitions of tetramethylammonium sulfate

Tetramethylammonium sulfate, [N(CH3)(4)](2)SO4, was prepared in high purity via ion exchange. Two reversible first-order phase transitions at 263 +/- 3 and 462 +/- 3 K have been established by differential thermal analysis and temperature-dependent X-ray powder diffraction. Crystal structure determinations at 223 and 293 K reveal completely ordered sulfate and tetramethylammonium tetrahedra for the low-temperature modification, while at room temperature half of the sulfate groups are orientationally disordered. The space groups are P4(2)/nmc with a = 7.5355 (9), c = 10.9910 (14) Angstrom and Z = 2 [wR(F-2) = 0.089 for 239 independent reflections] and P4/nbm with a = 10,8948 (9), c = 10.759 (2) Angstrom and Z = 4 [wR(F-2) = 0.118 for 792 independent reflections], respectively. Powder patterns, as recorded at 473 IC. by the Guinier-Simon X-ray technique, show that the high-temperature modification is cubic face-centered with a = 11.026 (2) Angstrom and is likely to contain only free-rotating tetrahedra. The common aristotype of all three modifications is the fluorite-type structure. An examination of group-subgroup relations is made.