  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 I2/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.
