Structural phase transitions in the perovskite-type layer compound NH3(CH 2)5NH3CdCl4 - Archive ouverte HAL
Article Dans Une Revue Journal de Physique Année : 1989

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

P. Negrier
M. Couzi
  • Fonction : Auteur
N.B. Chanh
  • Fonction : Auteur
C. Hauw
  • Fonction : Auteur
A. Meresse
  • Fonction : Auteur

Résumé

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.
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Dates et versions

jpa-00210925 , version 1 (04-02-2008)

Identifiants

Citer

P. Negrier, M. Couzi, N.B. Chanh, C. Hauw, A. Meresse. Structural phase transitions in the perovskite-type layer compound NH3(CH 2)5NH3CdCl4. Journal de Physique, 1989, 50 (4), pp.405-430. ⟨10.1051/jphys:01989005004040500⟩. ⟨jpa-00210925⟩

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