Phases, kinetics and structure of alkali-C 60 compounds: preparation of Rb 3- and (Rb 3− xK x)-C 60 superconductors

Abstract

System exploration of the compound-formation kinetics, phase composition and super-conductivity of Rb:C 60 mixtures, as diagnosed by X-ray crystallography and shielding diamagnetism measurements, reveals a single superconducting phase: f.c.c. Rb 3C 60 compound (a = 14.43 A ̊ , T c = 29.6 K) analogous to f.c.c. K 3C 60 (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. Rb 6C 60, analogous to the A 6C 60 compounds (A =Cs, K) reported earlier, but there is also a b.c.t. Rb 4C 60 compound and indications of another phase intermediate between C 60 and Rb 3C 60. 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 Rb 2K 1C 60 and Rb 1K 2C 60 ( T c values near 28 and 22.5 K, respectively). In contrast to the always sharp onset for Rb 3C 60, these compounds exhibit the rounded onsets characteristic of residual inhomogeneity.