NIR FT Raman study of phase transitions in ferroelectric pyridinium periodate

NIR FT Raman study has been performed for pyridinium periodate, a new room temperature ferroelectric, over a wide range of temperatures. The existence of two solid–solid phase transitions was confirmed. The molecular dynamics examined for the first time of both (C5H6N)+ cation and (IO4)− anion was shown to be evidently related to the observed transitions. The temperature dependences of some characteristic frequencies of both ions point to a kind of coupling between them. The critical behaviour of the internal vibrations of the tetrahedral anion reflecting partially the critical behaviour of the noncentrosymmetrical pyridinium cation was revealed. The cation–anion coupling is considered to be responsible for the long-range ordering leading to the appearance of ferroelectricity for the compound under study.