Phases, kinetics and structure of alkali-C₆₀ compounds: preparation of Rb₃- and (Rb_3-xK_x)-C₆₀ superconductors

System exploration of the compound-formation kinetics, phase composition and super-conductivity of Rb:C₆₀ mixtures, as diagnosed by X-ray crystallography and shielding diamagnetism measurements, reveals a single superconducting phase: f.c.c. Rb₃C₆₀ compound (a = 14.43 A ̊, T_c = 29.6 K) analogous to f.c.c. K₃C₆₀ (a = 14.26 A ̊, T_c = 19.2 K). The formation kinetics are interpreted in terms of staging, i.e., segregation and growth of several phase-separated compounds. The other prominent phase is b.c.c. Rb₆C₆₀, analogous to the A₆C₆₀ compounds (A =Cs, K) reported earlier, but there is also a b.c.t. Rb₄C₆₀ compound and indications of another phase intermediate between C₆₀ and Rb₃C₆₀. In samples prepared from mixtures in the 2.2:1 to 2.8 range, the shielding reaches a maximum of >50%, a high value for powder samples, and the X-ray pattern is free of interfering phases. Also reported are indications of two mixed-alkali phases prepared in high yield and tentatively associated with Rb₂K₁C₆₀ and Rb₁K₂C₆₀ (T_c values near 28 and 22.5 K, respectively). In contrast to the always sharp onset for Rb₃C₆₀, these compounds exhibit the rounded onsets characteristic of residual inhomogeneity.