Spin–vibronic superexchange and dynamical vibronic order in ammoniated cubic fullerides

In the Mott–Hubbard cubic fulleride Li3(NH3)6C60 the superexchange energy is found to be much smaller than the rotational quantum for Jahn–Teller deformations at fullerene sites. This gives rise to a new type of superexchange interaction involving threefold degenerate vibronic ground states of C603- ions. Comparison of the spin–vibronic spectrum for a dimer fragment [C603-]2 of a b.c.c. lattice with the spin–orbital spectrum of a corresponding dimer containing threefold orbitally degenerate sites (t2g1-t2g1 model) exhibits drastic differences. In contrast to spin–orbital models, the spin–vibronic superexchange can only be antiferromagnetic and shows significant vibronic reduction of superexchange amplitude in agreement with magnetic susceptibility data. In function of transfer parameters two quadrupolar fully dynamical vibronic orders with quenched vibronic moments on sites develop in the ground state. The spectrum of collective vibronic excitations is calculated and the stability of vibronic ordered phases is discussed.