Novel phase behavior in normal alkanes

Abstract

X-ray scattering studies on aligned films of binary mixtures of the normal alkanes ${\mathrm{C}}_{23}$${\mathrm{H}}_{48}$ and ${\mathrm{C}}_{28}$${\mathrm{H}}_{58}$ reveal, for the first time in such materials, the existence of a new equilibrium phase having the symmetry of a smectic crystal, possibly a hexatic. This phase occurs between the hexagonally packed ${\mathit{R}}_{\mathrm{II}}$ and the lower-temperature orthorhombic ${\mathit{R}}_{\mathrm{I}}$, plastic crystalline, layered, rotator phases. We argue that this loss of order is due to local distortion fluctuations in the hexagonal phase. Furthermore, we have identified an ABC-to-ABAB restacking transition within the ordered ${\mathit{R}}_{\mathrm{II}}$ phase.