Packing models for high-pressure polymeric phases of C-60

Packing models of three polymeric phases of C-60 obtained through pressure- temperature treatments have been reconsidered through a lattice energy mini mization method using atom-atom potentials. Orthorhombic phase O', previous ly assigned Immm symmetry, is best described by space group Pmnn. Whereas t he Immm model contains one orientation of the polymeric chains, the Pmnn mo del presents two orientations related by a glide plane. For the tetragonal phase, two packing patterns of polymeric tetragonal layers are possible: on e with Immm symmetry represents a pseudotetragonal packing of translational ly identical adjacent layers; the second packing pattern, actually tetragon al (P4(2)/mmc), consists of two layers related by a 4(2) screw axis and is similar to 4 kJ mol(-1) more stable. Concerning phase R, three packing mode s of hexagonal layers are possible; the previously published structure of p hase R corresponds to the least stable packing. Thus, the previous structur al descriptions should be reexamined. (C) 1998 Academic Press.