Pressure-induced order-disorder phase transition in superconducting CaC6
  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. 
A. Gauzzi, N. Bendiab, M. d'Astuto, B. Canny, M. Calandra, F. Mauri,   G. Loupias, N. Emery, C. Herold, P. Lagrange, M. Hanf...