Phase instabilities in KC60

The high-temperature f.c.c. phase of KC60 is unstable upon cooling and transforms into the orthorhombic polymer (OP). We show in this paper that the polymeric phase is also unstable above room temperature and separates into the thermodynamically stable K3C60 + α-C60 phases. The analysis of the lifetime of both f.c.c. and polymeric phases, probed by ESR as a function of temperature, allows the determination of activation energies of both f.c.c. → polymeric and polymeric → K3C60 + α-C60 phase transformations, as being 0.17 and 1.1 eV, respectively.