A vibrational study of phase transitions in (CH3)4NNO3

The crystal structure of (CH 3 ) 4 NNO 3 at 303 K is tetragonal (D 4h 7 (P4/nmm)) with Z=2 and the nitrate ions are disordered. Differential scanning calorimetry data revealed the phase sequence. The Raman and infrared spectra (10-5000 cm -1 ) of single crystals and polycrystalline samples of (CH 3 ) 4 NNO 3 were investigated in the temperature range 20-305 K. The assignments of most of the bands are proposed. A systematic temperature dependence study of the frequencies and band widths was carried out. The results show the existence of three phases in accordance with the calorimetric data. The vibrational analysis of the cations shows that they are slightly distorted in phase I, whereas the distortion is larger in phase III. The Raman spectra indicate disorder of the anions in phase I. The phase transitions are caused by the freezing-out of disorder of the anions and increased distortion of the cations.