Intercalation of ammonia into K₃C₆₀

Abstract

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 ∼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 ∼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.