Structure and Superconductivity in Alkali-Ammonia Complex Fullerides

Alkali ammonia complex fullerides form a rich variety of compounds with unique structural and electronic properties. Intercalation of NH3 into K3C60 produces orthorhombic NH3C60, which undergoes a metal-insulator transition instead of superconductivity that is expected to occur at around 30 K from the empirical relation between Tc and the lattice parameter. Synthesis in liquid ammonia yields new superconducting fullerides (NH3)xNaA2C60 (0.5 < x < 1, A = K, Rb). While these compounds retain an fcc cell, they exhibit anomalous dependence of Tc on the lattice. These two examples suggest that the off-centred cations in the octahedral site have a serious effect on superconductivity.