Structural phase transitions in CaSi₂ under high pressure

In situ 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 regularl̊y 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.