Displacement disorder in a liquid crystalline phase with cubic symmetry

Abstract

The cubic phase of the lyotropic system hexa-ethyleneglycol mono n-dodecyl ether (C 12 EO 6 )/water is studied by small angle X-ray scattering, at the synchrotron radiation facility of LURE. The indexation of 15 Bragg reflections is strongly in favor of a body-centred cubic structure with space group Ia3d. The presence of diffuse scatterings in reciprocal space demonstrates the existence of large fluctuations of position in the crystal. The organization of these diffuse scatterings indicates that these fluctuations correspond to a linear disorder caused by displacements along directions. These displacements are discussed within the framework of the structural model usually proposed for Ia3d cubic phases. Their existence can be understood considering the usual mechanical properties of liquid crystals Etude par diffusion des rayons X aux petits angles de la phase cubique du systeme lyotrope mono n-dodecyl ether hexa-ethyleneglycol (C 12 EO 6 )/eau. Proposition d'une structure cubique centree dont le groupe d'espace est Ia3d. Existence d'importantes fluctuations de position dans le cristal, correspondant a un desordre lineaire du a des deplacements le long des directions