Molecular-Dynamics in Perfluoro-n-Eicosane. 1. Solid-Phase Behavior and Crystal-Structures

Abstract

Perfluoro‐n‐eicosane (n‐C20F42) passes through three different solid modifications, called M(T<146 K), I(146<T<200 K), and R(T>200 K). Structures of the phases M and R were characterized by x‐ray scattering experiments. Crystals are built up by rectangular layers. In modification M layers are set up by arrays of right‐handed and left‐handed (15/7‐) helices in alternate succession. The unit cell is monoclinic (a=9.65 Å, b=5.70 Å, c=28.3 Å, and β=97°; space group Pa). In modification R  molecules are highly mobile. There is no ordering with regard to the helix handedness. Molecules resemble cylindrical objects and are packed in a hexagonal dense mode. Regular layer stacking results in crystals with rhombohedral symmetry (a=b=5.70 Å, c=85.00 Å, γ=120°; space group R-3m).