RAMAN-SPECTROSCOPIC EVIDENCE OF THE PHASE-TRANSITION IN CH3NH3HGCL3

Raman scattering in methylammonium mercury trichloride crystals is measured at 300 and 343 K. IR absorption of polycrystalline samples is recorded at 300 K in the range 200 to 4000 cm-1 to facilitate the identification of vibrational bands. The most thermosensitive band in the Raman spectrum, identified with the C-N symmetric stretching mode, is singled out for temperature variation study. From the nature of change of frequency, intensity, and width of this band with increase in temperature it is concluded that the crystal undergoes a first order phase transition around 330 K (57-degrees-C) making a change over from the ferroelectric to the paraelectric state. An attempt to explain the mechanism of the phase transition is made in terms of disorder of the constituent molecular ions.