Bulk superconductivity at 38 K in a molecular system

C<sub>60</sub>based solids<sup>1</sup> are archetypal molecular superconductors with transition temperatures (T<sub>c</sub>) as high as 33 K (refs 2–4). T<sub>c</sub> of face-centred-cubic (f.c.c.) A<sub>3</sub>C<sub>60</sub> (A=alkali metal) increases monotonically with inter C<sub>60</sub> separation, which is controlled by the A<sup>+</sup> cation size. As Cs<sup>+</sup> is the largest such ion, Cs<sub>3</sub>C<sub>60</sub> is a key material in this family. Previous studies revealing trace superconductivity in Cs<sub>x</sub>C<sub>60</sub> 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<sub>3</sub>C<sub>60</sub> can be reproducibly isolated by solvent-controlled synthesis and has the highest T<sub>c</sub> of any molecular material at 38 K. In contrast to other A<sub>3</sub>C<sub>60</sub>materials, two distinct cubic Cs<sub>3</sub>C<sub>60</sub> structures are accessible. Although f.c.c. Cs<sub>3</sub>C<sub>60</sub> 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<sub>3</sub>C<sub>60</sub> from insulating at ambient pressure to superconducting without crystal structure change and reveals a broad maximum in T<sub>c</sub> at []7 kbar. We attribute the observed T<sub>c</sub> maximum as a function of inter C<sub>60</sub>separation—unprecedented in fullerides but reminiscent of the atom-based cuprate superconductors—to the role of strong electronic correlations near the metal–insulator transition onset