X-ray study of the first-order phase transition Pcab–Bmab in (CH3CH2NH3)2CdCl4

The structural first-order phase transition of the layer structure (CP2H5NH3)2CdCl4 at 216 K is studied by single-crystal X-ray diffraction. The low-temperature phase (OLT) belongs to the space group Pcab, whereas the room-temperature phase (ORT) belongs to the space group Bmab. In both structures, the hydrogen atoms are found from electron density maps. and their positions refined for the OLT phase. The organic residues are hydrogen-bonded to the layers formed by the corner-linked octahedra through the three H atoms of the ammonium ions. Due to the center of symmetry which lies in the center of the octahedra, the chains are placed on both sides of the layers. The free end of the alkyl chain is directed towards the inter-layer space, linking the next layer by van der Waals forces. The ORT phase has a disordered structure containing two statistically distributed and symmetry-equivalent chain positions. At the transition temperature Bmab-Pcab only one position of the chain remains, inducing thus a rearrangement of the structure characterised by the primitive space group symmetry.