TbFe10Si2 crystallizes in the orthorhombic Immm space group. Tb(1) is bonded in a 20-coordinate geometry to two equivalent Fe(1), two equivalent Fe(2), four equivalent Fe(3), eight equivalent Fe(4), and four equivalent Si(1) atoms. There are four inequivalent Fe sites. In the first Fe site, Fe(1) is bonded in a 14-coordinate geometry to one Tb(1), one Fe(1), two equivalent Fe(3), four equivalent Fe(2), four equivalent Fe(4), and two equivalent Si(1) atoms. In the second Fe site, Fe(2) is bonded in a 14-coordinate geometry to one Tb(1), one Fe(2), two equivalent Fe(3), four equivalent Fe(1), four equivalent Fe(4), and two equivalent Si(1) atoms. In the third Fe site, Fe(3) is bonded in a 12-coordinate geometry to two equivalent Tb(1), two equivalent Fe(1), two equivalent Fe(2), four equivalent Fe(4), and two equivalent Si(1) atoms. In the fourth Fe site, Fe(4) is bonded to two equivalent Tb(1), two equivalent Fe(1), two equivalent Fe(2), two equivalent Fe(3), two equivalent Fe(4), and two equivalent Si(1) atoms to form distorted FeTb2Fe8Si2 cuboctahedra that share corners with four equivalent Si(1)Tb2Fe10 cuboctahedra, corners with ten equivalent Fe(4)Tb2Fe8Si2 cuboctahedra, edges with two equivalent Si(1)Tb2Fe10 cuboctahedra, edges with four equivalent Fe(4)Tb2Fe8Si2 cuboctahedra, faces with four equivalent Si(1)Tb2Fe10 cuboctahedra, and faces with six equivalent Fe(4)Tb2Fe8Si2 cuboctahedra. Si(1) is bonded to two equivalent Tb(1), two equivalent Fe(1), two equivalent Fe(2), two equivalent Fe(3), and four equivalent Fe(4) atoms to form distorted SiTb2Fe10 cuboctahedra that share corners with six equivalent Si(1)Tb2Fe10 cuboctahedra, corners with eight equivalent Fe(4)Tb2Fe8Si2 cuboctahedra, edges with three equivalent Si(1)Tb2Fe10 cuboctahedra, edges with four equivalent Fe(4)Tb2Fe8Si2 cuboctahedra,  a faceface with one Si(1)Tb2Fe10 cuboctahedra, and faces with eight equivalent Fe(4)Tb2Fe8Si2 cuboctahedra.
TbFe10Si2 crystallizes in the orthorhombic Immm space group. Tb(1) is bonded in a 20-coordinate geometry to two equivalent Fe(1), two equivalent Fe(2), four equivalent Fe(3), eight equivalent Fe(4), and four equivalent Si(1) atoms. Both Tb(1)-Fe(1) bond lengths are 3.02 Å. Both Tb(1)-Fe(2) bond lengths are 2.94 Å. All Tb(1)-Fe(3) bond lengths are 2.94 Å. All Tb(1)-Fe(4) bond lengths are 3.18 Å. All Tb(1)-Si(1) bond lengths are 3.09 Å. There are four inequivalent Fe sites. In the first Fe site, Fe(1) is bonded in a 14-coordinate geometry to one Tb(1), one Fe(1), two equivalent Fe(3), four equivalent Fe(2), four equivalent Fe(4), and two equivalent Si(1) atoms. The Fe(1)-Fe(1) bond length is 2.33 Å. Both Fe(1)-Fe(3) bond lengths are 2.68 Å. All Fe(1)-Fe(2) bond lengths are 2.91 Å. All Fe(1)-Fe(4) bond lengths are 2.56 Å. Both Fe(1)-Si(1) bond lengths are 2.59 Å. In the second Fe site, Fe(2) is bonded in a 14-coordinate geometry to one Tb(1), one Fe(2), two equivalent Fe(3), four equivalent Fe(1), four equivalent Fe(4), and two equivalent Si(1) atoms. The Fe(2)-Fe(2) bond length is 2.41 Å. Both Fe(2)-Fe(3) bond lengths are 2.66 Å. All Fe(2)-Fe(4) bond lengths are 2.56 Å. Both Fe(2)-Si(1) bond lengths are 2.52 Å. In the third Fe site, Fe(3) is bonded in a 12-coordinate geometry to two equivalent Tb(1), two equivalent Fe(1), two equivalent Fe(2), four equivalent Fe(4), and two equivalent Si(1) atoms. All Fe(3)-Fe(4) bond lengths are 2.43 Å. Both Fe(3)-Si(1) bond lengths are 2.63 Å. In the fourth Fe site, Fe(4) is bonded to two equivalent Tb(1), two equivalent Fe(1), two equivalent Fe(2), two equivalent Fe(3), two equivalent Fe(4), and two equivalent Si(1) atoms to form distorted FeTb2Fe8Si2 cuboctahedra that share corners with four equivalent Si(1)Tb2Fe10 cuboctahedra, corners with ten equivalent Fe(4)Tb2Fe8Si2 cuboctahedra, edges with two equivalent Si(1)Tb2Fe10 cuboctahedra, edges with four equivalent Fe(4)Tb2Fe8Si2 cuboctahedra, faces with four equivalent Si(1)Tb2Fe10 cuboctahedra, and faces with six equivalent Fe(4)Tb2Fe8Si2 cuboctahedra. Both Fe(4)-Fe(4) bond lengths are 2.37 Å. Both Fe(4)-Si(1) bond lengths are 2.39 Å. Si(1) is bonded to two equivalent Tb(1), two equivalent Fe(1), two equivalent Fe(2), two equivalent Fe(3), and four equivalent Fe(4) atoms to form distorted SiTb2Fe10 cuboctahedra that share corners with six equivalent Si(1)Tb2Fe10 cuboctahedra, corners with eight equivalent Fe(4)Tb2Fe8Si2 cuboctahedra, edges with three equivalent Si(1)Tb2Fe10 cuboctahedra, edges with four equivalent Fe(4)Tb2Fe8Si2 cuboctahedra,  a faceface with one Si(1)Tb2Fe10 cuboctahedra, and faces with eight equivalent Fe(4)Tb2Fe8Si2 cuboctahedra.