Phase transitions inKC60: Dimer formation via rapid quenching

A metastable orthorhombic form of ${\mathrm{KC}}_{60}$ is identified from synchrotron x-ray powder-diffraction measurements. Structure analysis reveals ${\mathrm{C}}_{60}$ dimers with a 9.34 Å center-to-center separation. This structure competes with equilibrium phase separation (α-${\mathrm{C}}_{60}$ plus ${\mathrm{K}}_3$ ${\mathrm{C}}_{60}$) and with polymer formation; it is obtained by quenching from above 420 K to 298 K and immediately cooling below 270 K. If the polymer is indeed a one-dimensional metal, the dimer structure should be a Peierls insulator.