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 T_c and the lattice parameter. Synthesis in liquid ammonia yields new superconducting fullerides (NH₃)_{cursive chi}NaA₂C₆₀ (0.5 < cursive chi < 1, A = K, Rb). While these compounds retain an fcc cell, they exhibit anomalous dependence of T_c on the lattice. These two examples suggest that the off-centred cations in the octahedral site have a serious effect on superconductivity.