The recently discovered structural transition in polymerized ${\mathrm{KC}}_{60}$ at about 50 K results in a doubling of the unit cell volume and accompanies the metal-insulator transition. Here we show that the $(\overrightarrow{a}+\overrightarrow{c},\overrightarrow{b},\overrightarrow{a}-\overrightarrow{c})$ superstructure results from small orientational charge density waves along the polymer chains and concomitant displacements of the surrounding ${\mathrm{K}}^{+}$ ions. The effect is specific for the space group $\mathrm{Pmnn}$ of ${\mathrm{KC}}_{60}$ and is absent in ${\mathrm{RbC}}_{60}$ and ${\mathrm{CsC}}_{60}$ (space group $I2/m).\mathrm{}$ The mechanism is relevant for the metal-insulator transition.

• Received 2 May 2002

DOI:https://doi.org/10.1103/PhysRevB.66.165425