Alkali Metal Fullerides
An established feature of superconductivity in alkali fullerides is the existence of a simple relationship between Tc and the cubic lattice constant, a, namely: Tc increases monotonically with a (see Fig. below). This simple empirical rule has motivated efforts to synthesise fulleride salts with expanded lattice dimensions via co-intercalation of small molecules like NH3 in order to drive Tc even higher. Ammoniation of superconducting alkali fullerides of stoichiometry A2A'C60 has led to diverse responses in their structural, conducting and magnetic properties.
Upon ammoniation, the Tc of Na2CsC60 increases significantly from 10.5 to 29.6 K whereas K3C60 superconductor becomes and antiferromanetic insulator with a metal-insulator transition at 40 K. The ultimate goal will be to prepare compositions of the general type (NH3)xNa2AC60 (A= K, Rb). By varying both the ammonia content x and selecting the alkali metal A we can adjust the lattice size in order to explore the phase diagram of the alkali fullerides close to the boundary between the superconducting to metal to insulator regions.
