Structural investigation of trimethylammonium tetrachloromercurate

Abstract

The temperature dependence of the crystal structure of TrMA 2 HgCl 4 (TrMA=trimethylammonium) is studied by single-crystal and powder X-ray diffraction between 161 and 373 K . Below room temperature, we did not find significant changes of the crystal structure. The phase transition at 325 K is described by a rotation of one of the two independent TrMA + cations by 40° around the N–H axis, resulting in major rearrangements in the network of hydrogen bonds. A large change of the volume of the unit cell is found at the phase transition at 357 K , resulting in damage to the single crystals. Powder diffraction then shows that this transition can be described as a α – β transition between the α and β -K 2 SeO 4 structure types. Structure refinements employing alternatively restrictions by the space group and additional restrictions by the C 3 non-crystallographic site symmetry for the TrMA + cations show that significant deviations from C 3 symmetry of the TrMA + cations do not exist at any temperature. The molecular dynamics and orientational disorder were studied by the maximum entropy method applied to the X-ray data. The diffraction results are corroborated by temperature dependent NMR experiments.