DIFFERENTIAL SCANNING CALORIMETRIC X-RAY-DIFFRACTION AND SPECTROSCOPIC STUDIES OF PHASE-TRANSITIONS IN THE BIDIMENSIONAL COMPOUND (C9H19NH3)(2)PBCL4

Four crystalline phases (called III, II, II' and I) have been observed in the perovskite-type layer compound (C9H19NH3)(2)PbCl4 using differ ential scanning calorimetry, x-ray diffraction and spectroscopic studi es. The crystallographic evolution with increasing temperature appears to be triclinic III --> monoclinic II --> monoclinic I. An intermedia te disordered phase II', stable in a narrow temperature range and stru cturally similar to phase II, has also been observed, such that the tr ansformation II --> I proceeds, in fact, two steps II --> II' --> I. P hase III, stable at low temperature, has an ordered structure. The pha se transition III --> Ii, which is of first-order type, corresponds to an order-disorder mechanism involving the organic part of the structu re (alkylammonium chains) and deformation of PbCl6 octahedra. The phas e transition II --> II', which is of second-order type, is related to the arrangement of PbCl6 octahedra of perovskite layers. The II' --> I transition is of first-order type and involves conformational disorde r of the chains. In phase I chain melting sets in.