By means of an approach that employs alkali-metal alloys, bulk single-phase (RbxK1-x)3C60 superconductors have been prepared for all x between 0 and 1. For x = 1 it is shown that the maximum superconducting fraction, which approaches 100% in sintered pellets, occurs at a Rb to C60 ratio of 3:1. More importantly, single-phase superconductors are formed at all intermediate values of x, and it is shown that the transition temperature (Tc) increases linearly with x in this series of materials. The formation of a continuous range of solid solutions demonstrates that the rubidium- and potassium-doped C60 superconducting phases must be isostructural, and furthermore, suggests that the linear increase in Tc with x results from a chemical pressure effect.
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