Bulk superconductivity at 38 K in a molecular system

C 60 -based solids are archetypal molecular superconductors with transition temperatures (Tc) as high as 33 K (refs2-4). Tc of face-centred-cubic (f.c.c.) A 3 C 60 (A=alkalimetal) increases monotonically with inter C 60 separation, which is controlled by the A + cation size. As Cs + is the largest such ion, Cs 3 C 60 is a key material in this family. Previous studies revealing trace superconductivity in Cs x C 60 materials have not identified the structure or composition of the superconducting phase owing to extremely small shielding fractions and low crystallinity. Here, we show that superconducting Cs 3 C 60 can be reproducibly isolated by solvent-controlled synthesis and has the highest Tc of any molecular material at 38 K. In contrast to other A 3 C 60 materials, two distinct cubic Cs 3 C 60 structures are accessible. Although f.c.c. Cs 3 C 60 can be synthesized, the superconducting phase has the A15 structure based uniquely among fullerides on body-centred-cubic packing. Application of hydrostatic pressure controllably tunes A15 Cs 3 C 60 from insulating at ambient pressure to superconducting without crystal structure change and reveals a broad maximum in Tc at 7 kbar. We attribute the observed Tc maximum as a function of inter C 60 separationunprecedented in fullerides but reminiscent of the atom-based cuprate superconductorsto the role of strong electronic correlations near the metal-insulator transition onset.