Maximum Tc at the verge of a simultaneous order-disorder and lattice-softening transition in superconducting CaC6

In order to account for the large drop of superconducting critical temperature, T, and for the dramatic increase in residual resistivity, rho(0), previously reported in CaC6 at P-cr approximate to 9 GPa, we studied the room-temperature crystal structure of bulk CaC6 samples as a function of pressure up to 13 GPa by means of synchrotron x-ray diffraction in diamond anvil cells. At P-cr, we found no change of the trigonal R m space group symmetry, but a large increase in isothermal compressibility, kappa, from -0.0082 GPa(-1) to -0.0215 GPa(-1), accompanied by a large broadening of Bragg peaks. With both effects being reversible, it follows that superconductivity in CaC6 is maximized at the verge of a peculiar order-disorder phase transition concomitant to a large lattice softening. Space group symmetry considerations supported by ab initio calculations of the relaxed structure within the density functional theory lead us to conclude that the disordered phase is presumably characterized by a random off-centering of the Ca atoms in the ab plane with respect to the C honeycomb layers.