Theory of distinct crystal structures of polymerized fullerides AC(60), A = K, Rb, Cs: The specific role of alkalis

The polymer phases of AC(60) form distinct crystal structures characterized by the mutual orientations of the (C-60(-))(n) chains. We show that the di rect electric quadrupole interaction between chains always favors the ortho rhombic structure Pmnn with alternating chain orientations. However, the sp ecific quadrupolar polarizability of the alkali metal ions leads to an indi rect interchain coupling which favors the monoclinic structure 12/m with eq ual chain orientations. The competition between direct and indirect interac tions explains the structural difference between KC60 and RbC60, CsC60.