Evidence of a structural anomaly at 14 K in polymerised CsC60

We report the results of a high-resolution synchrotron X-ray powder diffraction study of polymerised CsC60 in the temperature range 4 to 40 K. Its crystal structure is monoclinic (space group I2/m), isostructural with RbC60. Below 14 K, a spontaneous thermal contraction is observed along both the polymer chain axis, a and the interchain separation along [111], d1. This structural anomaly could trigger the occurrence of the spin-singlet ground state, observed by NMR at the same temperature.