MgTb2Se4 crystallizes in the orthorhombic Pnma space group. Mg(1) is bonded in a trigonal pyramidal geometry to one Se(2), one Se(3), and two equivalent Se(1) atoms. There are two inequivalent Tb sites. In the first Tb site, Tb(1) is bonded in a 7-coordinate geometry to one Se(2), two equivalent Se(3), and four equivalent Se(1) atoms. In the second Tb site, Tb(2) is bonded in a 6-coordinate geometry to one Se(2), one Se(3), and four equivalent Se(1) atoms. There are three inequivalent Se sites. In the first Se site, Se(1) is bonded in a 5-coordinate geometry to one Mg(1), two equivalent Tb(1), and two equivalent Tb(2) atoms. In the second Se site, Se(2) is bonded in a T-shaped geometry to one Mg(1), one Tb(1), and one Tb(2) atom. In the third Se site, Se(3) is bonded in a rectangular see-saw-like geometry to one Mg(1), one Tb(2), and two equivalent Tb(1) atoms.
MgTb2Se4 crystallizes in the orthorhombic Pnma space group. Mg(1) is bonded in a trigonal pyramidal geometry to one Se(2), one Se(3), and two equivalent Se(1) atoms. The Mg(1)-Se(2) bond length is 2.46 Å. The Mg(1)-Se(3) bond length is 2.63 Å. Both Mg(1)-Se(1) bond lengths are 2.61 Å. There are two inequivalent Tb sites. In the first Tb site, Tb(1) is bonded in a 7-coordinate geometry to one Se(2), two equivalent Se(3), and four equivalent Se(1) atoms. The Tb(1)-Se(2) bond length is 2.85 Å. There is one shorter (2.81 Å) and one longer (2.82 Å) Tb(1)-Se(3) bond length. There are two shorter (3.11 Å) and two longer (3.24 Å) Tb(1)-Se(1) bond lengths. In the second Tb site, Tb(2) is bonded in a 6-coordinate geometry to one Se(2), one Se(3), and four equivalent Se(1) atoms. The Tb(2)-Se(2) bond length is 2.71 Å. The Tb(2)-Se(3) bond length is 2.86 Å. There are two shorter (2.95 Å) and two longer (2.99 Å) Tb(2)-Se(1) bond lengths. There are three inequivalent Se sites. In the first Se site, Se(1) is bonded in a 5-coordinate geometry to one Mg(1), two equivalent Tb(1), and two equivalent Tb(2) atoms. In the second Se site, Se(2) is bonded in a T-shaped geometry to one Mg(1), one Tb(1), and one Tb(2) atom. In the third Se site, Se(3) is bonded in a rectangular see-saw-like geometry to one Mg(1), one Tb(2), and two equivalent Tb(1) atoms.