RAMAN-STUDY OF LOW-TEMPERATURE PHASE-TRANSITIONS IN THE CUBIC PEROVSKITE CH3NH3PBCL3

Raman spectra of polycrystalline samples of CH3NH3PbCl3 were studied a t between 80 and 300 K. An assignment of most of the observed bands is proposed. The first-order phase transitions previously detected at 17 2 and 179 K were characterized: [GRAPHICS] The spectral evolution and the mechanism proposed are in agreement with the theoretical analysis of normal vibrations and with previous X-ray diffraction studies and d ielectric measurements. The I <-> II transition involves mainly confor mational and orientational motions of the organic cation, whereas the antiferroelectric transition at 172 K is connected with a complex mech anism involving the freezing of the cation motions coupled with the di stortion of the PbCl6 octahedra. A systematic measurement of the CH3NH 3+ torsional vibration (at 483 cm(-1)) was performed, and the activati on energy E-a = 10.1 kJ mol(-1) was extracted from the half-width vers us temperature curve.