Polymorphism control of superconductivity and magnetism in Cs₃ C₆₀ close to the Mott transition

The crystal structure of a solid controls the interactions between the electronically active units and thus its electronic properties. In the high-temperature superconducting copper oxides, only one spatial arrangement of the electronically active Cu ²⁺ units-a two-dimensional square lattice-is available to study the competition between the cooperative electronic states of magnetic order and superconductivity¹. Crystals of the spherical molecular C₆₀^3- anion support both superconductivity and magnetism but can consist of fundamentally distinct three-dimensional arrangements of the anions. Superconductivity in the A ₃ C₆₀ (A = alkali metal) fullerides has been exclusively associated with face-centred cubic (f.c.c.) packing of C₆₀ ^3- (refs 2, 3), but recently the most expanded (and thus having the highest superconducting transition temperature, T c; ref. 4) composition Cs ₃C₆₀ has been isolated as a body-centred cubic (b.c.c.) packing, which supports both superconductivity and magnetic order. Here we isolate the f.c.c. polymorph of Cs₃ C₆₀ to show how the spatial arrangement of the electronically active units controls the competing superconducting and magnetic electronic ground states. Unlike all the other f.c.c. A₃C₆₀ fullerides, f.c.c. Cs₃C ₆₀ is not a superconductor but a magnetic insulator at ambient pressure, and becomes superconducting under pressure. The magnetic ordering occurs at an order of magnitude lower temperature in the geometrically frustrated f.c.c. polymorph (Néel temperature T_N = 2.2 K) than in the b.c.c.-based packing (T_N = 46 K). The different lattice packings of C₆₀ ^3- change T c from 38 K in b.c.c. Cs ₃C₆₀ to 35 K in f.c.c. Cs₃C₆₀ (the highest found in the f.c.c. A₃C₆₀ family). The existence of two superconducting packings of the same electronically active unit reveals that T c scales universally in a structure-independent dome-like relationship with proximity to the Mott metal-insulator transition, which is governed by the role of electron correlations characteristic of high-temperature superconducting materials other than fullerides.