CRYSTAL DYNAMICS AND PSEUDO-SPIN THEORY ANALYSIS IN THE PEROVSKITE CH3NH3PBCL3

The cubic perovskite substance CH3NH3PbCl3 exhibits two low temperatur es phase transitions according , the phase sequence : III (P222(1)) <- - (172K) --> II (P4/mmm) <-- (179K) --> I (Pm3m) The three phases have been caracterized by Raman spectroscoy. The I <-> II transition invol ves mainly the rotational and orientational motions of the organic cat ion, whereas the antiferroelectric transition at 172K is connected wit h a complex mechanism involving the freezing of the cation motions cou pled with the distortion of the PbCl6 octahedra. This phase sequence h as been also analysed in terms of pseudo-spin theory.