Credit: © 2010 NPG

Alkali metal fullerides form cubic structures where the C60 molecule acts as an anion in face- or body-centred cubic lattices, and show superconducting behaviour at relatively high temperatures of around 30–40 K. The three-dimensional nature of these superconducting materials differs from the well-known cuprates, which superconduct through two-dimensional copper–oxygen layers. The fullerides therefore offer more scope to study the effect of crystal structure on superconducting properties.

Now, an international team led by Matthew Rosseinsky from the University of Liverpool and Kosmas Prassides from Durham University have shown1 how dependent the formation of superconducting states is on the spatial arrangement of the two ions. The team studied the face-centred cubic (fcc) polymorph of Cs3C60. Unlike other fcc alkali metal fullerides, and its body-centred (bcc) polymorph, it is not a superconductor at ambient pressure. The application of a few kilobars of pressure, however, leads to the compound becoming a superconductor at around 30 K.

The application of pressure leads to a reduced unit cell volume, fitting with the observation of superconductivity in other alkali metal fullerides with smaller volumes than Cs3C60. Plotting the temperature at which superconducting behaviour occurs (Tc) for a range of compounds and pressures against volume shows a distinct correlation, with a dome-shaped curve peaking around 760 Å3 per fulleride. Although the fullerides' cubic symmetry is the key to superconducting behaviour, it is the separation between the anions that determines Tc.