Structural phase transitions in [(CH3)4P]3M2Cl9 (M = Bi and Sb) hybrid compounds revealed by temperature-controlled Raman spectroscopy and X-ray diffraction

Two phosphonium chlorobismuthate (III) and chloroantimonate (III) crystals have been prepared and their thermal behaviors have been studied by thermal analyses, temperature-controlled Raman spectroscopy and temperature-controlled X-ray diffraction. Refinements of the diffraction pattern, carried out by the Le Bail method, confirm the high purity of both samples. Raman spectroscopy revealed a decomposition of the compounds above 250 degrees C, later confirmed by energy dispersive X-ray (EDX) analysis and X-ray powder diffraction (XRPD). In addition, temperature-controlled Raman spectroscopy and X-ray powder diffraction allowed us to reveal the existence of transitions in the 25-150 degrees C temperature range, which had previously gone completely unnoticed despite numerous published studies. These transitions are related to reorientations of the tetramethyl phosphonium cations [(CH3)4P]+ and rotations of [M2Cl9]3- (M = Bi or Sb) dimers in the structure. The rearrangements of the cationic and anionic entities then change the thermal expansion coefficients measured by diffraction along the trigonal crystallographic axes of the structure.