Intermolecular overlap geometry gives two classes of fulleride superconductor:: Electronic structure of 38 K Tc Cs3C60

Superconductivity emerges for the A15 polymorph of the fulleride Cs(3)C(60) upon compression to a pressure of similar to 4 kbar. Using density functional theory we study the bonding in the A15 phase as a function of unit cell volume comparing it to that in the fcc polymorph. We find that, despite its smaller packing density, the bcc-derived A15 phase has both a substantially wider bandwidth for the partially occupied t(1u) band and a higher density of states at the Fermi level. This result can be traced to the striking differences in the nature of the interanion overlap in the A15 and fcc structures showing that the A15 Cs(3)C(60) phase is not simply yet another superconducting fulleride albeit with a very high T(c)-the two sphere packings (body centered versus face centered) observed experimentally produce two electronically distinct classes of fulleride superconductors.