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 tet
ragonal phases of CaSi2 at room temperature. Lattice parameters of the tetr
agonal phase regularly change with increasing pressure while the rhombohedr
al phase undergoes two structural phase transitions: A trigonal structure i
s 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 transi
tion towards the AlB2-like phase that differs from the low-pressure phases
by the almost planar bonds of silicon,