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 fur the long-range ordering leading to the appearance of ferroelectricity for the compound under study. (C) 2001 Elsevier Science Ltd. All rights reserved.