DIFFERENTIAL SCANNING CALORIMETRIC, X-RAY-DIFFRACTION, AND SPECTROSCOPIC STUDIES OF PHASE-TRANSITIONS IN THE 2-DIMENSIONAL COMPOUND (C6H13NH3)(2)PBCL4

Five structural phase transitions have been observed in the perovskite -type layer compound (C6H13NH3)(2)PbCl4 using differential scanning ca lorimetry, X-ray diffraction, and vibrational analysis. Phase IV. stab le at low temperature, has an ordered structure. The phase transition IV --> III', which is of first-order type, corresponds to an order-dis order mechanism involving the organic part of the structure (alkylammo nium chains) coupled with deformation of PbCl6 octahedra. The phase tr ansition III' --> III, which is of second-order type, is related to th e arrangement of the mineral matrix (octahedra of perovskite layers). The sequence of transitions III --> II' --> II is characterized by a w eak thermal anomaly corresponding to a second-order or nearly second-o rder transformation. It is suggested that phase II' could be incommens urate. The II --> I transition is of first-order type and involves con formational disorder of the chains. In the phase I chain melting sets in, it mainly consists in the introduction of kinks GTG' in the alkyla mmonium ions.