DSC, X-RAY-DIFFRACTION AND RAMAN STUDIES OF HIGH-TEMPERATURE PHASE-TRANSITIONS IN THE PEROVSKITE-TYPE LAYER COMPOUND BUTYLENEDIAMMONIUM LEAD TETRACHLORIDE

Butylenediammonium lead tetrachloride exhibits three structural phase transitions according to the sequence I - (324 K) --> gamma - (341 K) --> beta - (420 K) --> alpha. These transitions are characterized by D SC measurements and Raman scattering on polycrystalline samples. The s pace groups and the cell parameters of I, gamma and beta phases were d etermined by x-ray diffraction on single crystals and powder samples. The three phases I, gamma and beta have the same space group P2(1)/c w ith Z = 4. The first-order phase transition at 324 K is driven by orie ntational and conformational motions of the butylenediammonium chains. The transition at 341 K is also of first-order type and involves both organic cation motions and PbCl6 octahedra deformations. The transiti on at 420 K was evidenced by Raman scattering; it leads to a plastic p hase similar to that observed in the homologous monoammonium compound (C3H7NH3)2PbCl4. This transitional behaviour and the crystal dynamics will be discussed and compared with those of homologous compounds.