Tb8CoGa3 crystallizes in the hexagonal P6_3mc space group. There are four inequivalent Tb sites. In the first Tb site, Tb(1) is bonded to one Co(1) and four equivalent Ga(1) atoms to form distorted TbGa4Co trigonal pyramids that share corners with two equivalent Tb(4)Ga6 octahedra, corners with three equivalent Tb(2)Ga3Co tetrahedra, corners with six equivalent Tb(1)Ga4Co trigonal pyramids,  an edgeedge with one Tb(2)Ga3Co tetrahedra, edges with four equivalent Tb(1)Ga4Co trigonal pyramids, and faces with two equivalent Tb(4)Ga6 octahedra. The corner-sharing octahedral tilt angles range from 54-58°. In the second Tb site, Tb(2) is bonded to one Co(1) and three equivalent Ga(1) atoms to form distorted TbGa3Co tetrahedra that share corners with six equivalent Tb(4)Ga6 octahedra, corners with nine equivalent Tb(1)Ga4Co trigonal pyramids, and edges with three equivalent Tb(1)Ga4Co trigonal pyramids. The corner-sharing octahedral tilt angles range from 16-47°. In the third Tb site, Tb(3) is bonded in a 3-coordinate geometry to one Co(1) and four equivalent Ga(1) atoms. In the fourth Tb site, Tb(4) is bonded to six equivalent Ga(1) atoms to form distorted TbGa6 octahedra that share corners with six equivalent Tb(2)Ga3Co tetrahedra, corners with six equivalent Tb(1)Ga4Co trigonal pyramids, faces with two equivalent Tb(4)Ga6 octahedra, and faces with six equivalent Tb(1)Ga4Co trigonal pyramids. Co(1) is bonded in a 7-coordinate geometry to one Tb(2), three equivalent Tb(1), and three equivalent Tb(3) atoms. Ga(1) is bonded in a 11-coordinate geometry to one Tb(2), two equivalent Tb(4), four equivalent Tb(1), and four equivalent Tb(3) atoms.
Tb8CoGa3 crystallizes in the hexagonal P6_3mc space group. There are four inequivalent Tb sites. In the first Tb site, Tb(1) is bonded to one Co(1) and four equivalent Ga(1) atoms to form distorted TbGa4Co trigonal pyramids that share corners with two equivalent Tb(4)Ga6 octahedra, corners with three equivalent Tb(2)Ga3Co tetrahedra, corners with six equivalent Tb(1)Ga4Co trigonal pyramids,  an edgeedge with one Tb(2)Ga3Co tetrahedra, edges with four equivalent Tb(1)Ga4Co trigonal pyramids, and faces with two equivalent Tb(4)Ga6 octahedra. The corner-sharing octahedral tilt angles range from 54-58°. The Tb(1)-Co(1) bond length is 2.85 Å. There are a spread of Tb(1)-Ga(1) bond distances ranging from 2.96-3.82 Å. In the second Tb site, Tb(2) is bonded to one Co(1) and three equivalent Ga(1) atoms to form distorted TbGa3Co tetrahedra that share corners with six equivalent Tb(4)Ga6 octahedra, corners with nine equivalent Tb(1)Ga4Co trigonal pyramids, and edges with three equivalent Tb(1)Ga4Co trigonal pyramids. The corner-sharing octahedral tilt angles range from 16-47°. The Tb(2)-Co(1) bond length is 2.66 Å. All Tb(2)-Ga(1) bond lengths are 3.11 Å. In the third Tb site, Tb(3) is bonded in a 3-coordinate geometry to one Co(1) and four equivalent Ga(1) atoms. The Tb(3)-Co(1) bond length is 2.80 Å. There are two shorter (3.19 Å) and two longer (3.73 Å) Tb(3)-Ga(1) bond lengths. In the fourth Tb site, Tb(4) is bonded to six equivalent Ga(1) atoms to form distorted TbGa6 octahedra that share corners with six equivalent Tb(2)Ga3Co tetrahedra, corners with six equivalent Tb(1)Ga4Co trigonal pyramids, faces with two equivalent Tb(4)Ga6 octahedra, and faces with six equivalent Tb(1)Ga4Co trigonal pyramids. There are three shorter (3.27 Å) and three longer (3.41 Å) Tb(4)-Ga(1) bond lengths. Co(1) is bonded in a 7-coordinate geometry to one Tb(2), three equivalent Tb(1), and three equivalent Tb(3) atoms. Ga(1) is bonded in a 11-coordinate geometry to one Tb(2), two equivalent Tb(4), four equivalent Tb(1), and four equivalent Tb(3) atoms.