X-ray and neutron powder diffraction from Rb3C60(NH3)x and Li3C60(NH3)x

Samples of Rb3C60(NH/D3)x and one of Li2·8C60(NH3)3·7 were prepared and examined by X-ray and neutron powder diffraction. A pattern of the lithium compound can be fitted to an Im[image omitted]m (a = 11·843(2) Å) cell in which there are orientationally disordered C60 molecules and the lithium and ammonia are coordinated together and occupy the tetrahedral holes. However, some patterns from samples of different history and heat treatment show extra reflection peaks. They can be indexed as a P[image omitted]m1 supercell of the Im[image omitted]m cubic cell. The Rb3C60(NH/D3)x patterns are more complex but can be indexed using two commensurate modulations. The relative amount in any one sample of these two commensurate modulations vary depending on method of preparation, annealing and sample ageing. The modulations are conceptually derived from a hypothetical Im[image omitted]m (a = 12·10 Å) cell. However, the modulations are so large in amplitude that the observed intensities no longer resemble the pattern from a hypothetical Im[image omitted]m structure. Their position, intensity and the number of observed peaks correspond more closely to those for a hypothetical icosahedral quasicrystal or icosahedral glass. Although this correspondence may not be close we believe that there is a tendency for the C60 molecules to become icosahedrally coordinated in these disordered systems.