INTERCALATION OF AMMONIA INTO K3C60

THE superconducting transition temperatures (T(c)s) in the alkali-metal-doped C60 superconductors (A3C60) seem to be determined by the size of the face-centred cubic (f.c.c.) unit cell, which in turn is dependent largely on the size of the metal cations occupying the interstitial sites1. We have shown previously2 that intercalation of ammonia into Na2CsC60 expands the unit cell and thereby increases T(c) by approximately 20 K. This has prompted us to explore the generality of this effect by attempting to expand the lattice of A3C60 superconductors containing larger cations. Here we show that intercalation of ammonia into K3C60 (which has a T(c) of approximately 19 K) yields (NH3)K3C60, which has a structure that can be described as an orthorhombic distortion of the f.c.c. structure with one K and one NH3 per octahedral site. Despite strong evidence that the extent of charge transfer from K to C60 is unchanged by ammonia intercalation, the compound is not superconducting. We believe that the absence of superconductivity is a consequence of electron localization resulting from the structural distortion. Thus the change in symmetry of the unit cell, as well as the cell size, may be an important factor in attempts to create C60-based superconductors with high T(c)s.