X-Ray Scattering Study of Restacking Transitions in the Crystalline-B Phases of Heptyloxybenzylidene Heptylaniline (7O.7)

Abstract

This paper reports a comprehensive x-ray diffraction study of the structure and the restacking transitions which occur within the crystalline-B phases of N-[4-(n-heptyl)oxybenzylidene]-4’-(n-heptyl)aniline (7O.7). Between (63^{\circ}C) and (59^{\circ}C), this system exhibits a rich structural behavior with a series of three transitions involving restacking and subtle changes in the intralayer packing. In order of decreasing temperature, the structures are hexagonal close packed ((69–63^{\circ}C)), orthorhombic F ((63–60.1^{\circ}C)), monoclinic C ((60.1–59.75^{\circ}C)), and simple hexagonal ((59.75–55^{\circ}C)). In the last three phases, there is also a long-wavelength modulation ((\lambda \sim 100\mathring{A})) in which the smectic layers are displaced along their normals. The wave vector of the modulation lies in the plane of the smectic layers directed along a reciprocal-lattice vector and exhibits a weak ((\sim 20%)) temperature variation. Both low-resolution (rotating-anode) and high-resolution (synchrotron) measurements of the structures and of the diffuse scattering are reported.