The polymer phases of $A{\mathrm{C}}_{60}$ form distinct crystal structures characterized by the mutual orientations of the $({\mathrm{C}}_{60}^{-}{)}_n$ chains. We show that the direct electric quadrupole interaction between chains always favors the orthorhombic structure $\mathrm{Pmnn}$ 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 $I2\mathrm{}/m$ with equal chain orientations. The competition between direct and indirect interactions explains the structural difference between ${\mathrm{KC}}_{60}$ and ${\mathrm{RbC}}_{60}$, ${\mathrm{CsC}}_{60}$.

• Received 4 April 2000

DOI:https://doi.org/10.1103/PhysRevLett.85.3197