STRUCTURES AND PHASES OF SUPERCONDUCTING ALKALI-METAL DOPED C-60

A simple model for the energetics of solid doped AxC60 is sufficient t o understand the evolution of structures for 0 less-than-or-equal-to x less-than-or-equal-to 6. The contributions found to be most important are hard-sphere packing constraints, the van der Waals energy, the Ma delung energy, and geometric relationships between the atomic arrangem ents on the surface of the icosahedron and neighboring C60 molecules a nd dopants. At x = 3 the A 15 structure is energetically competitive, despite its larger atomic volume, and is favored for higher values of x since it has more sites available for dopants. Analysis of the avail able experimental data and the well-known near instability of the A 15 structure suggest that the superconductivity in KxC60 for x near 3 is influenced by a connected but poorly ordered metastable matrix with t he A 15 structure, which may be difficult to detect using conventional crystallographic techniques. The present model explains the large jum p in Meissner volume at x = 3, the negative sign and large magnitude o f dT(c)/dp, and the Raman spectrum at x = 3. Similarities with structu res of other exotic materials also follow from the model.