fullerides

Fullerides are a class of chemical salts in which fullerene molecules, most commonly C60, accept electrons donated by metal cations to form ionic compounds. The general formula MxC60 describes salts in which x corresponds to the number of electrons transferred per C60 molecule; alkali metals and sometimes alkaline-earth metals are common dopants. The C60 unit becomes a C60n− anion, and the cations occupy interstitial sites in a crystal lattice, typically an fcc arrangement of C60 with the metal ions in octahedral or tetrahedral voids.

The best-studied fullerides are alkali-doped C60, such as KC60, RbC60, Cs3C60, and related compounds. In these materials, electrons populate the t1u-derived bands derived from the LUMO of C60. The balance between electron–electron interactions and electron–phonon coupling yields a range of electronic states, including metallic, insulating, and superconducting phases. A3C60 salts are notably metallic and can exhibit superconductivity at low temperatures; the transition temperature varies with composition and can be influenced by applied pressure, with some phases showing enhanced Tc under pressure. Other stoichiometries, such as A4C60 and A6C60, tend to be insulating because of full band filling and correlation effects.

Synthesis methods commonly involve reacting C60 with alkali metals at elevated temperature or via diffusion in evacuated or inert environments, and with solvated routes using liquid ammonia or similar solvents. Variants include solvated fullerides and higher-pressure or polymeric phases, as well as films and composites.

Historically, the study of fullerides intensified after the discovery of fullerenes, with superconductivity in A3C60 salts reported in the early 1990s, spurring ongoing research into their structure, bonding, and potential applications.