STRUCTURE AND SUPERCONDUCTIVITY IN ALKALI-AMMONIA COMPLEX FULLERIDES

Alkali ammonia complex fullerides form a rich variety of compounds wit h unique structural and electronic properties. Intercalation of NH3 in to K3C60 produces orthorhombic NH3C60, which undergoes a metal-insulat or transition instead of superconductivity that is expected to occur a t around 30 K from the empirical relation between T-c and the lattice parameter. Synthesis in liquid ammonia yields new superconducting full erides (NH3)(x)NaA(2)C(60) (0.5 < x < 1, A = K, Rb). While these compo unds retain an fee cell, they exhibit anomalous dependence of T-c on t he lattice. These two examples suggest that the off-centred cations in the octahedral site have a serious effect on superconductivity. (C) 1 997 Elsevier Science Ltd.