Phase transition in the compound NH3(CH2)5NH3TlCl5

Penthylene diammonium pentachlorothallate(III) exhibits a phase transition at 316 K This transition has been characterized by optical birefringence, dielectric measurements differential scanning calorimetry and spectroscopic measurements on polycrystalline samples. The space group and the cell parameters of phase I were determined by X-ray diffraction from single crystals. Phase I has space group P212121, with Z = 4, a = 7.696(3), b = 13.2890(17) and c = 13.503(18) A. The transition is both displacive and order-disorder involving mainly conformational changes of the organic chain [NH3(CH2)5NH3]2+ coupled with distortion of the TlCl6 octahedra. This behaviour and the crystal dynamics will be discussed and compared with those found in the literature for homologous compounds.