Phase transitions in CH<inf>3</inf>NH<inf>3</inf>C1O<inf>4</inf> and CH<inf>3</inf>ND<inf>3</inf>ClO<inf>4</inf>

Infrared and Raman spectra (10–5000 cm-1) of CH3NH3C]O4 and CH3ND3C1O4 were investigated in the temperature range 90–470 K. Single crystal Raman spectral studies were also carried out at 310 and 90 K. Assignments of the measured frequencies to the internal modes have been made. Based on a systematic temperature dependent study of widths and frequencies of prominent bands, the phase transitions at 321 and 451 K are characterized. In phase III (below 321 K), the cations and anions show splittings due to inequivalent sites. In phase II (between 321 and 451 K), the results show the absence of inequivalent sites and large motional freedom of the anion. In phase I (above 451 K), the cations and anions undergo isotropic reorientational motions. © 1992, Taylor & Francis Group, LLC. All rights reserved.