X-ray scattering study of restacking transitions in the crystalline-Bphases 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 °C and 59 °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 °C), orthorhombic F (63–60.1 °C), monoclinic C (60.1–59.75 °C), and simple hexagonal (59.75–55 °C). In the last three phases, there is also a long-wavelength modulation (λ∼100 Å) 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 (∼20%) temperature variation. Both low-resolution (rotating-anode) and high-resolution (synchrotron) measurements of the structures and of the diffuse scattering are reported.