Theory of Distinct Crystal Structures of Polymerized Fullerides AC60,   A=K, Rb, Cs: the Specific Role of Alkalis
  The polymer phases of AC60 form distinct crystal structures characterized by the mutual orientations of the (C60-)n chains. We show that the direct electric quadrupole interaction between chains always favors the orthorhombic structure Pmnn with alternating chain orientations. However the specific quadrupolar polarizability of the alkali metal ions leads to an indirect interchain coupling which favors the monoclinic structure I2/m with equal chain orientations. The competition between direct and indirect interactions explains the structural difference between KC60 and RbC60, CsC60. 
K.H. Michel and A.V. Nikolaev