Lattice dynamics and structural phase transitions in perovskite-type layer compounds. IV. Diffuse X-ray scattering in (CH₃NH₃)₂CdCl₄

For pt.III see ibid., vol.17, p.217 (1984). Phase transitions of (CH 3 NH 3 ) 2 CdCl 4 (MACC) have been investigated by means of diffuse X-ray scattering experiments. The diffuse scattering patterns observed in the disordered orthorhombic room temperature (ORT) and tetragonal low-temperature (TLT) phases have been interpreted through the hypothesis that disorder involves not only the methylammonium groups but also the CdCl 6 octahedra perovskite-type layers. A simple model of planar disorder qualitatively accounts for the observed diffuse patterns. The data indicate that each layer can be considered as a mosaic of ordered 'clusters' involving about a hundred octahedra, and that disorder occurs between layers along the direction c perpendicular to the layer planes. On going into the monoclinic low-temperature (MLT) phase, the diffuse scattering disappears as expected for an ordered phase.