Vibrational study of phase transitions in CH₃NH₃NO₃ and CH₃ND₃NO₃

Abstract

Abstract Raman and infrared spectra (10–5000 cm −1 ) of single crystals and polycrystalline samples of CH 3 NH 3 NO 3 and of its ND 3 deuterated derivative were investigated in the temperature range 30–400 K. A systematic temperature dependence study of the frequencies and band widths was performed. The results show the existence of three phases. Assignments of most of the bands observed are proposed. Raman spectra of single crystal CH 3 NH 3 NO 3 in the internal and lattice region are interpreted based on a D 2h 16 structure and relative polarization data. A thermal hysteresis characteristic of the first order was observed in the phase II⇌phase I transition. Raman and infrared spectral features of phase I indicate this to be a plastic phase, in which cations and anions exhibit isotropic reorientational motion. The spectral changes that occur in the phase II → phase III transition suggest freezing out of thermal motion and reveal only minor structural changes in phase III compared with phase II.