Tb5Si2B8 crystallizes in the tetragonal P4/mbm space group. There are two inequivalent Tb sites. In the first Tb site, Tb(1) is bonded in a 12-coordinate geometry to two equivalent B(3), three equivalent B(2), four equivalent B(1), and three equivalent Si(1) atoms. In the second Tb site, Tb(2) is bonded to two equivalent B(3) and four equivalent Si(1) atoms to form corner-sharing TbSi4B2 octahedra. The corner-sharing octahedral tilt angles are 57°. There are three inequivalent B sites. In the first B site, B(1) is bonded in a 9-coordinate geometry to four equivalent Tb(1), one B(2), two equivalent B(1), and two equivalent B(3) atoms. In the second B site, B(2) is bonded in a 9-coordinate geometry to six equivalent Tb(1), one B(2), and two equivalent B(1) atoms. In the third B site, B(3) is bonded in a 9-coordinate geometry to one Tb(2), four equivalent Tb(1), and four equivalent B(1) atoms. Si(1) is bonded in a 9-coordinate geometry to two equivalent Tb(2), six equivalent Tb(1), and one Si(1) atom.
Tb5Si2B8 crystallizes in the tetragonal P4/mbm space group. There are two inequivalent Tb sites. In the first Tb site, Tb(1) is bonded in a 12-coordinate geometry to two equivalent B(3), three equivalent B(2), four equivalent B(1), and three equivalent Si(1) atoms. Both Tb(1)-B(3) bond lengths are 2.71 Å. There are two shorter (2.76 Å) and one longer (2.97 Å) Tb(1)-B(2) bond length. There are two shorter (2.65 Å) and two longer (2.73 Å) Tb(1)-B(1) bond lengths. There is one shorter (3.08 Å) and two longer (3.14 Å) Tb(1)-Si(1) bond lengths. In the second Tb site, Tb(2) is bonded to two equivalent B(3) and four equivalent Si(1) atoms to form corner-sharing TbSi4B2 octahedra. The corner-sharing octahedral tilt angles are 57°. Both Tb(2)-B(3) bond lengths are 2.77 Å. All Tb(2)-Si(1) bond lengths are 2.92 Å. There are three inequivalent B sites. In the first B site, B(1) is bonded in a 9-coordinate geometry to four equivalent Tb(1), one B(2), two equivalent B(1), and two equivalent B(3) atoms. The B(1)-B(2) bond length is 1.76 Å. Both B(1)-B(1) bond lengths are 1.81 Å. Both B(1)-B(3) bond lengths are 1.83 Å. In the second B site, B(2) is bonded in a 9-coordinate geometry to six equivalent Tb(1), one B(2), and two equivalent B(1) atoms. The B(2)-B(2) bond length is 1.87 Å. In the third B site, B(3) is bonded in a 9-coordinate geometry to one Tb(2), four equivalent Tb(1), and four equivalent B(1) atoms. Si(1) is bonded in a 9-coordinate geometry to two equivalent Tb(2), six equivalent Tb(1), and one Si(1) atom. The Si(1)-Si(1) bond length is 2.35 Å.