Structural Investigations of the Phase Transitions of Tetramethylammonium Sulfate

Tetramethylammonium sulfate, [N(CH₃)₄]₂SO₄, 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 P4₂/nmc with a = 7.5355 (9), c = 10.9910 (14) Å and Z = 2 [wR(F²) = 0.089 for 239 independent reflections] and P4/nbm with a = 10.8948 (9), c = 10.789 (2) Å and Z = 4 [wR(F²) = 0.118 for 792 independent reflections], respectively. Powder patterns, as recorded at 473 K by the Guinier-Simon X-ray technique, show that the high-temperature modification is cubic face-centered with a = 11.026 (2) Å 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.

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