Structural phase transitions in CaSi2 under high pressure

In sial x-ray-diffraction measurements under high pressure (P up to 20 GPa) have been used to study structural changes in the rhombohedral and the tetragonal phases of CaSi2 at room temperature. Lattice parameters of the tetragonal phase regularly change with increasing pressure while the rhombohedral phase undergoes two structural phase transitions: A trigonal structure is observed for 9 < P < 16 GPa and a new AlB2-like phase appears for P > 16 GPa. These observations lead to ascribe the appearance of superconductivity to the trigonal phase while the highest T-c's can be related to the transition towards the AlB2-like phase that differs from the low-pressure phases by the almost planar bonds of silicon,