Reversible polymerization in doped fullerides under pressure: the case of C60(Fe(C5H5)2)2

High-pressure Raman studies have been carried out on single crystalline C(60)(Fc)(2) nanosheets up to 25.4 GPa. Our results show that the charge transfer between Fc (ferrocene) and C(60) increases in the low-pressure range. Above 5 GPa, C(60) molecules start to form a chainlike polymer structure, and this polymerization is reversible upon decompression, in contrast to that of pristine C(60). The special layered structure of C(60)(Fc)(2) restricts the polymerization of C(60) molecules in some directions and explains the formation of the linear chainlike polymeric structure of the C(60) lattice under pressure. We suggest that the reversible polymerization is related to the increased charge transfer and the overridden steric repulsion of counterions.