PHASE-TRANSITIONS AND PSEUDO-SPIN DESCRIPTION IN THE PEROVSKITE CH3NH3PBCL3

The differential scanning calorimetry study of the compound of formula CH3NH3PbCl3 in the temperature range 168 K-181 K shows two phase tran sitions according to the phase sequence: phase III (P222(1)) <-- (172K ) --> phase II (P4/mmm) <-- (179K) --> phase I (Pm3m) These transition s were characterized by X-ray diffraction, DSC and Raman scattering. T he I <-> II transition involves mainly the rotational and orientationa l motions of the organic cation, whereas the antiferroelectric transit ion at 172K is connected with a complex mechanism involving the freezi ng of the cation motions coupled with the distorsion of the PbCl6 octa hedra. This sequence has been also analysed in terms of pseudo-spin th eory.