Numéro
J. Phys. I France
Volume 1, Numéro 9, September 1991
Page(s) 1321 - 1333
DOI https://doi.org/10.1051/jp1:1991210
DOI: 10.1051/jp1:1991210
J. Phys. I France 1 (1991) 1321-1333

Inelastic neutron scattering investigation of low temperature phase transition in Rb 2ZnCl 4 and K 2ZnCl 4

M. Quilichini, V. Dvorák and P. Boutrouille

Laboratoire Léon Brillouin (CEA-CNRS), CEN-Saclay, 91191 Gif-sur-Yvette Cedex, France


(Received 5 February 1991, accepted in final form 16 May 1991)

Abstract
Inelastic scattering of neutrons has revealed soft optic modes at the T point $\frac{1}{2}({\bf b}^*+{\bf c}^*)$ of the Brillouin zone both in Rb 2ZnCl 4 and K 2ZnCl 4 which are responsible for the phase transition from the ferroelectric to the lowest temperature phase of these materials. Moreover, in K 2ZnCl 4 near the T point a minimum on the soft optic branch in the direction $\left(\mu{\bf b}^*+\frac{1}{2}{\bf c}^*\right)$ has been found which confirms the existence of a new incommensurate phase recently discovered by Gesi. The origin of this incommensurate phase is discussed from a phenomenological point of view and formulae for elastic constants are derived describing their behaviour near transition into incommensurate phase.

Résumé
Des mesures de diffusion inélastique des neutrons ont mis en évidence l'existence d'un mode optique mou au point T $\left(\frac{1}{2}({\bf b}^*+{\bf c}^*)\right)$ de la zone de Brillouin responsable de la transition de la phase ferroélectrique vers la phase basse température dans les deux composés Rb 2ZnCl 4 and K 2ZnCl 4. Pour K 2ZnCl 4 on montre que la branche optique molle présente un minimum au voisinage de T dans la direction $\left(\mu{\bf b}^*+\frac{1}{2}{\bf c}^*\right)$, ce qui confirme l'existence de la nouvelle phase incommensurable récemment trouvée par Gesi. L'origine de cette phase est discutée sur la base d'un modèle phénoménologique dont on dérive aussi les formules des constantes élastiques et leur comportement au voisinage de la transition vers la phase incommensurable.



© Les Editions de Physique 1991

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