PHASE-TRANSITIONS IN N(CH3)4HSO4 - A NOVEL COMPOUND WITH AN INCOMMENSURATE PHASE

Two phase transitions have been detected by calorimetric and optical studies in crystals of N(CH3)4HSO4. At T(i) = 232(2) K, the crystal undergoes a second-order orthorhombic to incommensurate transition and at T(c) = 202(2)K, a first-order incommensurate to monoclinic transition. The structures of the three phases have been studied by single-crystal X-ray diffraction. The orthorhombic normal phase, measured at room temperature, exhibits two orientations of the sulfate ions. In the incommensurate phase, the modulation vector is parallel to c* with q congruent-to 0.39c*; the two orientations of the sulfate are retained in this phase. The structure of the low-temperature phase was determined at 175 K; the orientational phenomenon disappears while twin domains are observed. Crystal data: tetramethylammonium hydrogen sulfate, C4H12N+ HO4S-, M(r) = 171.2. At room temperature: Pn2(1)a, a = 16.467(9), b = 7.543(6), c = 6.939(5) angstrom, Z = 4, lambda(Mo K-alpha) = 0.71069 angstrom, R = 0.055 for 931 reflections. At T = 215 K: P111BARBAR,Pn2(1)a a = 16.230(4), b = 7.581(2), c = 6.825(1) angstrom, Z = 4, lambda(Cu K-alpha) = 1.5418 angstrom, R = 0.047 for 647 main reflections. At T = 175 K: P2(1), a = 17.451(1), b = 7.595(1), c = 13.563(1) angstrom, beta = 112.140(2)-degrees, Z = 8, lambda(Cu K-alpha) = 1.5418 angstrom, R = 0.064 for 1680 reflections.