SYNTHESIS, SUPERCONDUCTIVITY AND ESR CHARACTERIZATION OF RB3C60 CRYSTALS

A one-step doping process of C60 crystals with three equivalents of ru bidium to produce Rb3C60 crystals was developed. The T(c)'s for all Rb 3C60 crystalline samples (aprproximately 30.5 K) were higher than thos e of the powder samples (28 to 29.6 K), but consistent with the report ed four-probe resistivity result for a doped C60 crystal (transition m idpoint = 30.2 K). High superconducting shielding fractions (between 6 0 and 90%) and sharp magnetic transition widths (DELTAT10-90 between a pproximately 3 K and 0.7 K) were observed for the samples doped with R b at temperatures of 300 to 450-degrees-C. The doping process, as well as the composition, of these samples were studied with the use of an ESR spectrometer. Two major components, RbC60 and Rb3C60, were quantit atively characterized. The superconducting shielding fractions of thes e samples were found to be linearly correlated with the Rb3C60 content obtained from ESR measurements. Higher-temperature doping ( > 400-deg rees-C) favored the formation of Rb3C60 at the expense of severe C60 c rystal fragmentation. The optimal doping condition to produce Rb3C60 c rystals was found to be around 300-degrees-C.