Instability of the doped high pressure rhombohedral C60 polymer structure

Despite the theoretical prediction of an interesting insulator-to-metal transition in rhombohedral C60 polymers (Rh-C60) upon electron doping, very few attempts have been performed up to now to actually intercalate it with donors. In this Letter we show that Rh-C60 converts to the monomer fcc C60 around 450 K; this explains the failure of previous doping attempts which required treatments at higher temperatures. Here we performed the Li doping of Rh-C60 in solution of liquid ammonia, a low temperature technique which avoids thermal depolymerisation. However, the rhombohedral structure is not retained after intercalation, yielding a monomer alkali-metal doped fulleride.