Packing models for high-pressure polymeric phases of C60

Packing models of three polymeric phases of C-60 obtained through pressure-temperature treatments have been reconsidered through a lattice energy minimization method using atom-atom potentials. Orthorhombic phase O', previously assigned Immm symmetry, is best described by space group Pmnn. Whereas the Immm model contains one orientation of the polymeric chains, the Pmnn model presents two orientations related by a glide plane. For the tetragonal phase, two packing patterns of polymeric tetragonal layers are possible: one with Immm symmetry represents a pseudotetragonal packing of translationally identical adjacent layers; the second packing pattern, actually tetragonal (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 modes of hexagonal layers are possible; the previously published structure of phase R corresponds to the least stable packing. Thus, the previous structural descriptions should be reexamined. (C) 1998 Academic Press.