Vibrational study of phase transitions in CH3NH3NO3 and CH3ND3NO3

Raman and infrared spectra (10–5000 cm−1) of single crystals and polycrystalline samples of CH3NH3NO3 and of its ND3 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 CH3NH3NO3 in the internal and lattice region are interpreted based on a D2h16 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.