Dimer to monomer phase transition in alkali-metal fullerides: Magnetic susceptibility changes

Ab initio calculations have been employed to investigate the peculiar chang e in magnetic property (from diamagnetic to paramagnetic) of the dianionic C-60-dimer phase in a rapidly cooled AC(60) samples (A: alkali metal). We f irst note that the triplet state of (C-60)(2)(-2) which was never considere d previously is nearly degenerate with the singlet state, and the transitio n barrier between the two states is reasonably small. This explains the sus ceptibility increase with an increase in temperature and the magnetic phase transition in the process of the dimer to monomer phase transition.