Contribution à l'étude structurale et spectroscopique du tétrachlorocadmiate de diéthylène triammonium et mise en évidence d'une transition de phase

Abstract : In this work is presented an experimental and theoretical study of (denH3)CdCl5 relative to physical, chemical and structural properties. It has been possible to obtain the following new results : the structure of the material is non-centrosymmetric ; it can be described as orthorhombic pseudo-Pnma=D16#2h, very close to the pseudo-tetragonal Pn4#2m=C4#4v structure, but its true symmetry is Pn2#1a=C9#2v. The interpretation of the Raman scattering spectra, namely as functions of temperature, has been possible with the help of an order-disorder concerning the orientation of the -NH3+ groups. The assignment of certain Raman scattering lines due to the lattice vibrations was possible, as well as of certain lines characteristic of the (denH3)3+ chain deformations or of the -NH3+ groups. This assignment, although partial, allows to separate the vibrational modes of the CdCl6 octahedra from the vibrational modes of the organic cation. A phase transition of second order with no hysteresis effect was determined. The transition temperature is Tc=168K. Below Tc the structure remains orthorhombic pseudo-tetragonal. This transition manifests by the presence of a central peak scattering corresponding to a dynamic relaxational process. The detailed study of the torsion mode of the -NH3+ group allowed to determine the hight of the double well potential barrier, the orientation correlation time, as well as the critical exponents characteristic of the long range order. The correlation length at room temperature was found to be of the order of 15 elementary cells. The study of the skeleton deformation mode (at 335 cm-1) showed that it is coupled to the jump motions of the -NH3+ groups. The linewidths analysis showed their double origin : dispersion of their frequency due to the desorder, anharmonicity (essentially cubic)
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Fouad Jebari. Contribution à l'étude structurale et spectroscopique du tétrachlorocadmiate de diéthylène triammonium et mise en évidence d'une transition de phase. Autre [cond-mat.other]. Université Paul Verlaine - Metz, 1994. Français. ⟨NNT : 1994METZ055S⟩. ⟨tel-01776809⟩

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