ALKALI-FULLERIDE SUPERCONDUCTORS - SYNTHESIS, COMPOSITION, AND DIAMAGNETIC SHIELDING

The recent report of a superconductivity onset near the critical temperature T(c) = 18 K in potassium-doped C60 raises questions concerning the composition and stability of the superconducting phase. The effects of mixing and heat treatment of K(x) C60 samples prepared over a wide range of initial compositions on the superconducting transition was determined from shielding diamagnetism measurements. A single superconducting phase (T(c) = 19.3 K) occurs for which the composition is K3C60. The shielding reaches a maximum of greater than 40 percent of the perfect diamagnetism, a high value for a powder sample, in samples prepared from 3:1 mixtures. A Rb(x)C60 sample prepared and analyzed in an analogous way exhibited evidence for superconductivity with T(c) = 30 K and a diamagnetic shielding of 7 percent could be obtained.