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 C603- 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 TN = 2.2 K) than in the b.c.c.-based packing (TN = 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.
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