Evidence of a structural anomaly at 14 K in polymerised CsC 60

We report the results of a high-resolution synchrotron X-ray powder diffraction study of polymerised CsC 60 in the temperature range 4 to 40 K. Its crystal structure is monoclinic (space group I 2/ m ), isostructural with RbC 60 . Below 14 K, a spontaneous thermal contraction is observed along both the polymer chain axis, a and the interchain separation along [111], d 1 . This structural anomaly could trigger the occurrence of the spin-singlet ground state, observed by NMR at the same temperature.