CaSi2 is hexagonal omega structure structured and crystallizes in the hexagonal P6/mmm space group. The structure is three-dimensional. Ca is bonded to twelve equivalent Si atoms to form a mixture of edge and face-sharing CaSi12 cuboctahedra. All Ca–Si bond lengths are 3.17 Å. Si is bonded in a 9-coordinate geometry to six equivalent Ca and three equivalent Si atoms. All Si–Si bond lengths are 2.36 Å.