Vibrational study of the ferroelastic phase transition in [(CH3)4P]3[Bi2Cl9] and [(CH3)4P]3[Bi2Br9]

Internal vibration modes of the [(CH3)4P]3[Bi2Cl9] and [(CH3)4P]3[Bi2Br9] crystals have been studied through their low temperature paraelastic-ferroelastic phase transitions at 151 and 205 K, respectively, using infrared spectroscopy. The studies show that the vibrational states of tetramethyphosphonium cations change markedly through the phase transitions. This is in line with the earlier suggested order-disorder mechanism of the low temperature transition.