  The recently discovered structural transition in polymerized KC60 at about 50
K results in a doubling of the unit cell volume and accompanies the
metal-insulator transition. Here we show that the (a+c,b,a-c) superstructure
results from small orientational charge density waves along the polymer chains
and concomitant displacements of the surrounding K+ ions. The effect is
specific for the space group Pmnn of KC60 and is absent in Rb- and CsC60 (space
group I2/m). The mechanism is relevant for the metal-insulator transition.
