Structural phase transitions in the perovskite-type layer compound NH3(CH 2)5NH3CdCl4

The perovskite-type layer compound NH3(CH2) 5NH3CdCl4 exhibits two structural phase transitions observed at Tc1 = 337 K and T2 = 417 K, respectively. These phase changes are studied by means of X-ray diffraction, X-ray diffuse scattering and Raman scattering experiments. Evidence is provided for the existence of a conformational disorder of the pentylenediammonium chains between « trans » and « twisted » states, superimposed on orientational disorder ; the « twisted » states of the chains are less stable than the « trans » ones and are thermally activated. The second order transition at Tc1 (Pnam ⇔ Imam) is due to orientational ordering of the « trans » chains ; the residual disorder observed well below T c1 is attributed to the « twisted » chains which remain orientationally disordered. The system progressively gets ordered at low temperature, as the « twisted » chains are dying out by thermal effect. The first order transition at T2 (Imam ⇔ C2/m) is driven by an increasing proportion of « twisted » chains with increasing temperature ; the monoclinic phase stable above T2 contains « twisted » chains only, arranged in an orientationally disordered manner. A phenomenological description of this phase sequence is proposed, based on a Landau-type development of the free energy, in terms of pseudo-spin variables derived from a multidimensional pseudo-spin model that describes orientational disorder of the pentylenediammonium chains.