Structural phase transitions inCaSi2under 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 ${\mathrm{CaSi}}_2$ 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 $9<P<\mathrm{}16\mathrm{}\mathrm{GPa}$ and a new ${\mathrm{AlB}}_2$-like phase appears for $P>\mathrm{}16\mathrm{}\mathrm{GPa}.$ These observations lead to ascribe the appearance of superconductivity to the trigonal phase while the highest $T_c\mathrm{’}\mathrm{s}$ can be related to the transition towards the ${\mathrm{AlB}}_2$-like phase that differs from the low-pressure phases by the almost planar bonds of silicon.