Theoretical model for the structural phase transition at the metal-   insulator transition in polymerized KC60
  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. 
B. Verberck, A.V. Nikolaev, and K.H. Michel