Phase transitions in KC60: Dimer formation via rapid quenching.

A metastable orthorhombic form of KC[sub 60] is identified from synchrotron x-ray powder-diffraction measurements. Structure analysis reveals C[sub 60] dimers with a 9.34 A center-to-center separation. This structure competes with equilibrium phase separation ([alpha]-C[sub 60] plus K[sub 3]C[sub 60]) and with polymer formation; it is obtained by quenching from above 420 K to 298 K and [ital immediately] cooling below 270 K. If the polymer is indeed a one-dimensional metal, the dimer structure should be a Peierls insulator.