JAHN-TELLER DISTORSION AND PHASE-TRANSITIONS IN THE ALKALI FULLERIDES

In icosahedral symmetry, the LUMO of A(n)C(60) molecule is a three-fol d degenerate tl, orbital. For n equal 1 to 5, the partial filling of t he band should confer a metallic behaviour and provide the possibility of a superconducting ground state. This is in disagreement with exper imental data: only A(3)C(60) seems to exhibit superconductivity and so me examples of semiconducting compounds such as K4C60 or Rb4C60 prove the failure ?of this picture. This could be explained by a Jahn-Teller distorsion of the C-60 molecule which leads to a partial lifting of t he degeneracy of t(1 alpha) orbital and could induce strong electronic correlations. In this model, tendency to localisation should prevail for even values of n, with completely filled subbands, but a supercond uctivity coupling is expected for odd values. The superconducting tran sition temperature should strongly decrease away from n = 3 on both si des of this maximum.