In many crystals of cubic and trigonal symmetry, the unpaired electrons at the unit cells reside in orbitally degenerate electronic states. This gives rise to spin-orbital exchange interaction between such magnetic centers (the Kugel-Khomskii model) which is often regarded as the origin of orbital order in staggered magnetic phases. Actually, in crystals with orbitally degenerate sites there is a much stronger interaction, the Jahn-Teller effect, which splits the degenerate orbitals on sites with a gap exceeding by orders of magnitude the intersite exchange interaction, reducing the latter to a conventional interaction between non-degenerate electronic states. Furthermore, when the effective elastic interaction between active JT distortions on neighbor sites is smaller than the separation of vibronic levels on sites, the JT effect renders dynamic and the exchange interaction occurs between degenerate vibronic levels. The resulting spin-vibronic exchange interaction together with the elastic interaction between active JT distortions gives rise to the ordering of vibronic states on sites (the vibronic order) which replaces the conventional orbital order in the KH model. The presence of spin-orbital interaction on the metal ions does not modify this picture if the ground spin-orbital multiplet is not a Kramers doublet.
In this talk, I will discuss the main features of spin-vibronic exchange interaction and vibronic order and their major differences from the orbital order and spin-orbital exchange interaction respectively. To avoid complications related to spin-orbit coupling, the insulating alkali doped fullerides Cs3C60 and Li3 (NH3)6C60 will be considered as examples.