YCo9Si4 crystallizes in the tetragonal I4/mcm space group. Y(1) is bonded in a 24-coordinate geometry to eight equivalent Co(1), eight equivalent Co(2), and eight equivalent Si(1) atoms. There are three inequivalent Co sites. In the first Co site, Co(1) is bonded to two equivalent Y(1), one Co(3), two equivalent Co(2), three equivalent Co(1), and four equivalent Si(1) atoms to form distorted CoY2Co6Si4 cuboctahedra that share  a cornercorner with one Co(3)Co8Si4 cuboctahedra, corners with eleven equivalent Co(1)Y2Co6Si4 cuboctahedra, faces with two equivalent Co(3)Co8Si4 cuboctahedra, and faces with seven equivalent Co(1)Y2Co6Si4 cuboctahedra. In the second Co site, Co(2) is bonded in a 11-coordinate geometry to two equivalent Y(1), one Co(3), two equivalent Co(1), two equivalent Co(2), and four equivalent Si(1) atoms. In the third Co site, Co(3) is bonded to four equivalent Co(1), four equivalent Co(2), and four equivalent Si(1) atoms to form a mixture of corner and face-sharing CoCo8Si4 cuboctahedra. Si(1) is bonded in a 11-coordinate geometry to two equivalent Y(1), one Co(3), four equivalent Co(1), and four equivalent Co(2) atoms.
YCo9Si4 crystallizes in the tetragonal I4/mcm space group. Y(1) is bonded in a 24-coordinate geometry to eight equivalent Co(1), eight equivalent Co(2), and eight equivalent Si(1) atoms. All Y(1)-Co(1) bond lengths are 3.29 Å. All Y(1)-Co(2) bond lengths are 3.13 Å. All Y(1)-Si(1) bond lengths are 3.22 Å. There are three inequivalent Co sites. In the first Co site, Co(1) is bonded to two equivalent Y(1), one Co(3), two equivalent Co(2), three equivalent Co(1), and four equivalent Si(1) atoms to form distorted CoY2Co6Si4 cuboctahedra that share  a cornercorner with one Co(3)Co8Si4 cuboctahedra, corners with eleven equivalent Co(1)Y2Co6Si4 cuboctahedra, faces with two equivalent Co(3)Co8Si4 cuboctahedra, and faces with seven equivalent Co(1)Y2Co6Si4 cuboctahedra. The Co(1)-Co(3) bond length is 2.39 Å. Both Co(1)-Co(2) bond lengths are 2.50 Å. There are two shorter (2.30 Å) and one longer (2.53 Å) Co(1)-Co(1) bond length. There are two shorter (2.47 Å) and two longer (2.53 Å) Co(1)-Si(1) bond lengths. In the second Co site, Co(2) is bonded in a 11-coordinate geometry to two equivalent Y(1), one Co(3), two equivalent Co(1), two equivalent Co(2), and four equivalent Si(1) atoms. The Co(2)-Co(3) bond length is 2.51 Å. Both Co(2)-Co(2) bond lengths are 2.57 Å. There are two shorter (2.29 Å) and two longer (2.42 Å) Co(2)-Si(1) bond lengths. In the third Co site, Co(3) is bonded to four equivalent Co(1), four equivalent Co(2), and four equivalent Si(1) atoms to form a mixture of corner and face-sharing CoCo8Si4 cuboctahedra. All Co(3)-Si(1) bond lengths are 2.32 Å. Si(1) is bonded in a 11-coordinate geometry to two equivalent Y(1), one Co(3), four equivalent Co(1), and four equivalent Co(2) atoms.