Sm5Si2B8 crystallizes in the tetragonal P4/mbm space group. There are two inequivalent Sm sites. In the first Sm site, Sm(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 Sm site, Sm(2) is bonded to two equivalent B(3) and four equivalent Si(1) atoms to form corner-sharing SmSi4B2 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 Sm(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 Sm(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 Sm(2), four equivalent Sm(1), and four equivalent B(1) atoms. Si(1) is bonded in a 9-coordinate geometry to two equivalent Sm(2), six equivalent Sm(1), and one Si(1) atom.
Sm5Si2B8 crystallizes in the tetragonal P4/mbm space group. There are two inequivalent Sm sites. In the first Sm site, Sm(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 Sm(1)-B(3) bond lengths are 2.76 Å. There are two shorter (2.81 Å) and one longer (3.03 Å) Sm(1)-B(2) bond length. There are two shorter (2.70 Å) and two longer (2.79 Å) Sm(1)-B(1) bond lengths. There is one shorter (3.16 Å) and two longer (3.20 Å) Sm(1)-Si(1) bond lengths. In the second Sm site, Sm(2) is bonded to two equivalent B(3) and four equivalent Si(1) atoms to form corner-sharing SmSi4B2 octahedra. The corner-sharing octahedral tilt angles are 57°. Both Sm(2)-B(3) bond lengths are 2.88 Å. All Sm(2)-Si(1) bond lengths are 2.97 Å. There are three inequivalent B sites. In the first B site, B(1) is bonded in a 9-coordinate geometry to four equivalent Sm(1), one B(2), two equivalent B(1), and two equivalent B(3) atoms. The B(1)-B(2) bond length is 1.79 Å. Both B(1)-B(1) bond lengths are 1.83 Å. Both B(1)-B(3) bond lengths are 1.84 Å. In the second B site, B(2) is bonded in a 9-coordinate geometry to six equivalent Sm(1), one B(2), and two equivalent B(1) atoms. The B(2)-B(2) bond length is 1.89 Å. In the third B site, B(3) is bonded in a 9-coordinate geometry to one Sm(2), four equivalent Sm(1), and four equivalent B(1) atoms. Si(1) is bonded in a 9-coordinate geometry to two equivalent Sm(2), six equivalent Sm(1), and one Si(1) atom. The Si(1)-Si(1) bond length is 2.35 Å.