X-RAY AND NEUTRON POWDER DIFFRACTION FROM RB3C60(NH3)(X) AND LI3C60(NH3)(X)

Samples of Rb3C60(NH/D-3)(x) and one of Li2.8C60(NH3)(3.7) were prepar ed and examined by X-ray and neutron powder diffraction. A pattern of the lithium compound can be fitted to an Im (3) over bar m (a = 11.843 (2) Angstrom) cell in which there are orientationally disordered C-60 molecules and the lithium and ammonia are coordinated together and occ upy the tetrahedral holes. However, some patterns from samples of diff erent history and heat treatment show extra reflection peaks. They can be indexed as a P (3) over bar m1 supercell of the Im (3) over bar m cubic cell. The Rb3C60(NH/D-3)(x) patterns are more complex but can be indexed using two commensurate modulations. The relative amount in an y one sample of these two commensurate modulations vary depending on m ethod of preparation, annealing and sample ageing. The modulations are conceptually derived from a hypothetical Im (3) over bar m (a = 12.10 Angstrom) cell. However, the modulations are so large in amplitude th at the observed intensities no longer resemble the pattern from a hypo thetical Im (3) over bar m structure. Their position, intensity and th e number of observed peaks correspond more closely to those for a hypo thetical icosahedral quasicrystal or icosahedral glass. Although this correspondence may not be close we believe that there is a tendency fo r the C-60 molecules to become icosahedrally coordinated in these diso rdered systems.