Infrared investigations of the order-disorder ferroelectric phase transitions in imidazolium halogenobismuthates(III) and halogenoantimonates(III): (C3N2H5)5Bi2Cl11, (C3N2H5)5Bi2Br11 and (C3N2H5)5Sb2Br11

Infrared spectra of the powdered (C3N2H5)(5)Bi2Cl11, (C3N2H5)(5)Bi2Br11 and (C3N2H5)(5)Sb2Br11 crystals in the region of internal vibrations of the imidazolium cations (3600 and 400 cm(-1)) at the temperature intervals of 10-300 K, covering paraelectric-ferroelectric phase transitions, are presented and discussed in this paper. The research shows that the vibrational states of the imidazolium cations change markedly during the paraelectric-ferroelectric phase transition. The continuous nature of these transitions is well reflected in the infrared spectra, which is consistent with the previous X-ray and dielectric findings. (C) 2009 Elsevier B.V. All rights reserved.