Electron-spin-resonance study of the dimer state ofKC60and its transformations

Recent x-ray results show that quenching the high-temperature fcc (rocksalt) phase of ${\mathrm{KC}}_{60}$ below 280 K produces a metastable orthorhombic structure in which the molecules exist as dimer pairs. Using electron-spin resonance, we find that this state is nonmagnetic and describe evidence for σ bonding. Upon heating, the dimer structure transforms first to the fcc phase at 280 K. The fcc phase is unstable at 300 K and transforms to the polymeric state at 340 K rather than undergoing equilibrium phase separation. We suggest that the transformation from dimer to fcc is driven by the onset of molecular orientational disorder.