  To establish the doping-dependent phase diagram in alkali-metal doped
C$_{60}$, we synthesize Rb-doped C$_{60}$ samples with different
stoichiometries by using the improved wet-chemistry technique. The doping
levels determined from the Raman scattering spectra often show the appearance
of three electrons corresponding to the band filling of three for the
synthesized compounds no matter matter what dopants are used. The multiple
phase coexistence with the unique Rb$_{3}$C$_{60}$ is identified from the
refined x-ray diffraction patterns. The phase fraction of Rb$_{3}$C$_{60}$ is
found to behave with the doping in a similar manor as the superconducting
shielding fraction. These rigorously established correlations among the
superconducting transition temperature along with the structural and phonon
vibrational properties allow us to single out Rb$_{3}$C$_{60}$ as the only
superconducting phase with the nearly constant transition temperature
regardless the doping level. These findings provide an experimental constraint
on the theory developments for the superconductivity in fullerides.
