  By means of synchrotron X-ray diffraction, we studied the effect of high
pressure, P, up to 13 GPa on the room temperature crystal structure of
superconducting CaC6. In this P range, no change of the pristine space group
symmetry, \textit{R\=3m}, is found. However, at 9 GPa, i.e. close to the
critical value at which a large T_c reduction was reported recently, we
observed a compressibility jump concomitant to a large broadening of Bragg
peaks. The reversibility of both effects upon depressurization and symmetry
arguments give evidence of an order-disorder phase transition of second order,
presumably associated with the Ca sublattice, which provides a full account for
the above Tc reduction.
