PHASE-TRANSITIONS AND CRYSTAL DYNAMICS IN THE CUBIC PEROVSKITE CH3NH3PBCL3

Raman spectra of polycrystalline samples of CH3NH3PbCl3 were studied a t temperature ranging from 80 K to 300 K. An assignment of most of the observed bands is proposed. The first order phase transitions previou sly detected at 172 and 179 K were characterized: III <-- (172 K) --> II <-- (179 K) --> I (P222(1)) (P4/mmm) (Pm3m) The spectral evolution and the mechanism proposed is in agreement with the theoretical analys is of normal vibrations and with previous X-ray diffraction studies an d dielectric measurements. The I <-> II transition involves mainly the conformational and orientational motions of the organic cation, where as the antiferroelectric transition at 172 K is connected with a compl ex mechanism involving the freezing of the cation motions coupled with the distorsion of the PbCl6 octahedra. A systematic measurement of th e CH3NH3+ torsional vibration (at 483 cm(-1)) was performed and the ac tivation energy E-a= 10.1 kJ mol(-1) was extracted from the half width vs temperature curve. (C) 1997 Elsevier Science Ltd.