a, The crystal structure of f.c.c. Cs₃C₆₀ (ORTEP representation). The C₆₀^3- anions (carbon, dark grey) are orientationally disordered—only one orientation is shown for clarity. The mean C₆₀^3--C₆₀^3- near-neighbour contacts (3.59 Å through neighbouring hexagon:pentagon C–C bonds) are markedly shorter than those in the lower-anion-density (817.9 ų per C₆₀^3-) A15 polymorph (3.80 Å through hexagon:hexagon faces: Supplementary Fig. 1b). The mean Cs…C distance (3.45 Å) for the tightly coordinated tetrahedral site of f.c.c. Cs₃C₆₀ is shorter than that in A15 Cs₃C₆₀ (3.66 Å). The more spacious octahedral Cs site in f.c.c. Cs₃C₆₀ supports a more expanded coordination environment—mean Cs…C distance, 3.90 Å. In common with all f.c.c. A₃C₆₀ phases²⁰, the Cs⁺ ion residing in the large octahedral interstices displays a very large isotropic thermal parameter, B_iso ( = 11.65(7) Ų), corresponding to a mean square fluctuation about the centre of the hole, = 0.665(2) Å. On cooling, the Debye–Waller factor decreases smoothly to low temperatures where considerable disorder (presumably of static origin) still persists ( = 0.373(1) Å at 30 K) (Supplementary Fig. 2a). The Cs cations (red) residing in the tetrahedral and octahedral interstices are shown with 90% thermal ellipsoids to emphasize this disorder, which is absent in A15 Cs₃C₆₀ where the Cs ions reside in less spacious distorted tetrahedral holes. b, Main panel, final observed (red) and calculated (blue line) synchrotron X-ray (λ = 0.40004 Å) powder diffraction profile for the f.c.c.-rich sample (85.88(2)%) at ambient temperature. The lower line (green) shows the difference profile, and the ticks show the reflection positions of the f.c.c. (top), A15 (3.31(5)%, upper middle), body-centred orthorhombic (b.c.o.) (6.7(2)%, lower middle), and CsC₆₀ (4.10(6)%, bottom) phases—refined parameters and agreement indices are given in Supplementary Table 1. Inset, expanded view of the diffraction profile (4.2-5.3°) with the observed reflections labelled by their (hkl) Miller indices.

a, Temperature dependence of the magnetic susceptibility, χ(T) (green circles), of f.c.c.-rich Cs₃C₆₀, obtained from the difference of the values at 5 T and 3 T. b, Temperature dependence of the spin–lattice relaxation rate, 1/¹³T₁. Inset, below 100 K, there is a distribution of relaxation rates, giving stretched-exponential behaviour to the ¹³C magnetization relaxation curves, M_z(τ) - M_z(0) ∝ exp[-(τ/T₁)^α], where M_z(τ) is the z-component of the nuclear spin magnetization measured at time τ after a train of pulses saturated the ¹³C nuclear spin magnetization. The stretch exponent α changes below ~15 K. c, Temperature dependence of the spin–spin relaxation rate, 1/¹³T₂. Gradual freezing of local magnetic moments gives a rapid increase below 15 K. d, ¹³³Cs NMR spectra of f.c.c. Cs₃C₆₀ between 4 and 65 K. O and T are resonances for the octahedral and tetrahedral sites, T′ and O′ are associated with the anion disorder²¹. e, Temperature dependence of the second moment, ¹³³M₂^1/2 (red circles). Its increase at low T is ascribed to the increased width of the distribution of static local magnetic fields (see also Supplementary Fig. 7). For comparison, we show ¹³³M₂^1/2 in A15 Cs₃C₆₀ (T_N = 46 K). f, Temperature evolution of the ZF μ⁺-spin polarization, P_μ(t), for f.c.c.-rich Cs₃C₆₀ between 0.625 and 100 K. At high temperatures, the spectra imply the presence of weak static nuclear moments together with a slow relaxation arising from fluctuating electronic moments. Between 16.2 and 2.5 K, they are consistent with inhomogeneous magnetism: that is, co-existing spin frozen and paramagnetic domains. Below 2.2 K, they are dominated by a short-lived heavily damped oscillating signal. g, Temperature evolution of the ZF μ⁺-spin precession frequency, ν_μ, below T_N ≈ 2.2 K, and of the volume fraction and relaxation rate, λ₁, of the slowly relaxing (paramagnetic) component for temperatures where paramagnetic and spin frozen domains coexist. The lines through the points are guides to the eye. a.u., arbitrary units. Error bars, ±s.d.

a, Temperature dependence of the magnetization, M (ZFC protocol, 20 Oe), at selected pressures. Inset, expanded view of the data near the onset of the metal-to-superconductor transition, T_c. FC and ZFC data confirming the Meissner effect are shown in Supplementary Fig. 9. b, Pressure dependence of T_c measured for three different f.c.c. Cs₃C₆₀ samples (blue/green/red circles). Filled (open) circles label data obtained with increasing (decreasing) pressure. T_c is defined as the temperature at which M(T) in a begins to decrease. Inset, evolution of the shielding fraction with change in pressure. The data for A15 Cs₃C₆₀ (squares) are also included⁵ to emphasize the lower pressure onset of bulk superconductivity in the f.c.c. polymorph. c, Pressure dependence of the unit cell volume of the f.c.c. Cs₃C₆₀ phase at 15 K up to an applied pressure of 13 GPa (different colours indicate different sample batches). A linear fit to the data up to 0.8 GPa gives a volume compressibility, κ = 0.053(1) GPa^-1, comparable to that measured for A15 Cs₃C₆₀ (κ = 0.054(5) GPa^-1)⁶. The line through the data points is a least-squares fit to the second-order Murnaghan equation of state, with an atmospheric pressure isothermal bulk modulus (K₀) of 13.7(3) GPa, and its pressure derivative K'₀ = 13.0(3).

a, Superconducting transition temperature, T_c, as a function of volume occupied per fulleride anion, V, at low temperature. The red/green/blue circles and pink squares correspond to the bulk T_c(V) behaviour observed in f.c.c.- and A15-structured Cs₃C₆₀, respectively. Open symbols represent data at pressures where trace superconductivity is observed and where in the A15 phase superconductivity coexists with antiferromagnetism. The yellow rhombi, dark blue triangles and brown inverted triangles correspond to the ambient pressure T_c of f.c.c. C₆₀^3- anion packings with Li₂CsC₆₀, Pa symmetry, and Fm m symmetry, respectively. b, Normalized superconducting transition temperature, T_c/T_c(max), as a function of the ratio (U/W) divided by the critical value (U/W)_c required to produce localization in the A₃C₆₀ fulleride structures with f.c.c.- and bcc-sphere packings. W_c is estimated as 0.55 eV for b.c.c.-structured A15 Cs₃C₆₀ and 0.35 eV for the f.c.c. phases. The symbols have the same meaning as in a. Inset, dependence of the t_1u conduction bandwidth on volume occupied per fulleride anion, V, for f.c.c.-sphere (red circles) and b.c.c.-sphere (pink squares) packings, as determined by electronic structure calculations¹².