First-Principles Prediction of High-Pressure Phase of CaC₆

The lattice dynamics of rhombohedral CaC₆ is studied as a function of pressure to probe its high pressure phase with low superconducting transition temperature using the density functional liner-response theory. The pressure-induced phase transition in CaC₆ is attributable to the softening transverse acoustic (TA) phonon mode at the zone boundary X (0.5, 0.0, 0.5) point. The high pressure phase is then explored by performing fully structural
optimization in the supercell which accommodates the atomic displacements corresponding to the eigenvectors of the unstable mode of TA(X). The high-pressure phase is predicted to be a monoclinic unit cell with space group P21/m.