(Rb_xK_1-x)₃C₆₀ superconductors: Formation of a continuous series of solid solutions

By means of an approach that employs alkali-metal alloys, bulk single-phase (Rb_xK_1-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 (T_c) 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 T_c with x results from a chemical pressure effect.