Vibrational study of the structural phase transitions in 4-aminopyridinium tetrachloroantimonate(III) (4-APCA) ferroelectric crystal by infrared spectroscopy

Infrared spectra of powdered 4-aminopyridinium tetrachloroantimonate(III) [4-NH2C5H4NH][SbCl4] (abbreviation: 4-APCA) in the region of the internal vibrations of the 4-aminopyridinium cation (3600 and 400 cm−1) and in the temperature interval 10-300 K covering three structural phase transitions are presented and discussed. The results confirm the order-disorder mechanism of the phase transition at 245 K. The paraelectric-ferroelectric phase transition at 271 K is hardly visible in the infrared spectra, which is in agreement with its “displacive” nature of the transition suggested by the single crystal X-ray and ac calorimetric studies.