  High quality Rb0.8Fe1.6+xSe2 single crystals are grown by a one step melting
growth method. Superconductivity has been observed in all samples with x more
than 0.05 and the maximum critical temperature Tc~32 K has been obtained in
samples with x from 0.1 to 0.2 from electronic resistivity measurement. A
superconducting dome in the electronic phase diagram of Tc as a function of Fe
content has been observed for the first time in this AxFeySe2 (A=K,Rb)
superconducting system. Two coexisting phases with structural modulation
vectors of q1=1/5 [3a*+b*] and q2=1/2[a*+b*] have been observed in all
superconducting samples by TEM observations. Moreover, phase separation in the
superconducting crystals, which can be recognized as micro-stripes in the ab
plane, are found to be highly dependent on the Fe content.
