Differential scanning calorimetric x-ray diffraction and spectroscopic studies of phase transitions in the bidimensional compound

Four crystalline phases (called III, II, and I) have been observed in the perovskite-type layer compound using differential scanning calorimetry, x-ray diffraction and spectroscopic studies. The crystallographic evolution with increasing temperature appears to be triclinic . An intermediate disordered phase II', stable in a narrow temperature range and structurally similar to phase II, has also been observed, such that the transformation proceeds, in fact, two steps . Phase III, stable at low temperature, has an ordered structure. The phase transition , which is of first-order type, corresponds to an order - disorder mechanism involving the organic part of the structure (alkylammonium chains) and deformation of octahedra. The phase transition , which is of second-order type, is related to the arrangement of octahedra of perovskite layers. The transition is of first-order type and involves conformational disorder of the chains. In phase I chain melting sets in.