MOLECULAR-DYNAMICS IN PERFLUORO-N-EICOSANE .1. SOLID-PHASE BEHAVIOR AND CRYSTAL-STRUCTURES

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 angstrom, b = 5.70 angstrom, c = 28.3 angstrom, and beta = 97-degrees; 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 angstrom, c = 85.00 angstrom, gamma = 120-degrees; space group R 3mBAR).