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)(x)NaA(2)C(60) (0.5 < x < 1, A = K, Rb). While these compounds retain an fee 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. (C) 1997 Elsevier Science Ltd.