Temperature dependence of the Raman spectra of NaClO₄ in relation to the 581 K phase transition

Abstract

The evolution of the internal and external modes in the Raman spectra of NaClO, has been studied in the neighbourhood of the transition temperature, at 581 K. Large changes in the frequencies of the external modes with temperature have been observed. These changes can be explained in terms of a model based on the reorientation of the perchlorate ions. INTRODUCIION A N b Y S I S OF THE NORMAL MODESMany authors have dealt with the vibrational spectra of perchlorates both in solution'" and as solids at room temperature and at low t e m p e r a t~r e .~,~ Studies of solid perchlorates have included both infrared ~pectra''~-' and Raman ~pectra.~'' In this paper, we report a study of the changes with temperature in the Raman spectra of anhydrous sodium perchlorate NaC104, from room temperature to the neighbourhood of the phase transition temperature T, = 308 "C. This is an order-disorder transition and is characterized by a disorder in the orientation of the ClOi ions. In the disordered hase, the structure is described by the space group Oh (Fm3m), if we do not consider the position of the oxygen atoms. Below T,, the structure is described by the space group 0:; (Cmcm). We shall show that this transition can be explained by the mean of a two-dimensional order parameter; and we shall give group theory arguments that satisfactorily explain the evolution of the phonon spectra.

Особенности Структурных Фазовых Переходов В Ионно-Молекулярных Кристаллах Перхлоратов

“…Исследование фазового перехода в перхлоратах важно для понимания механизма и моле-кулярной природы структурных перестроек в твердых телах [33]. В литературе мало данных, посвященных исследованиям именно этого класса ионных соедине-ний [34][35][36][37][38][39]. Поэтому исследование реориентационной подвижности и процессов разупорядочения анионов в области полиморфных превращений в перхлоратах щелочных металлов методом КР является актуальной задачей.…”