Charge transfer and polymer phases in AC60 (A=K, Rb, Cs) fullerides

Abstract

Starting from a microscopic model of interactions in A+C60− crystals, where A+ is an alkali metal ion and C60− the molecular ion, we present a theory of the phase transitions from the orientationally disordered Fm3̄m structure to polymer and dimer phases. The electronic charge transfer is accounted for by the t1u molecular orbitals of C60−. The resulting Coulomb interactions between neighboring C60− and between C60− and the sublattice of alkali metal ions lead to new orientation dependent potentials which have to be added to the intermolecular pair potential and to the crystal field. By studying the orientation dependence of the crystal field and the molecular field, we find that, in comparison with the phase transition Fm3̄m→Pa3̄ known from C60-fullerite, additional channels to a Pmnn structure with subsequent polymerization and dimerization are opened. We study the symmetry of the low temperature phases and compare with experimental results. The present work comprises an extension of the formalism of rotator functions to molecular orbitals.