MgTb2Se4 is Spinel-like structured and crystallizes in the orthorhombic Pnma space group. Mg(1) is bonded to one Se(2), one Se(3), and two equivalent Se(1) atoms to form MgSe4 tetrahedra that share corners with two equivalent Tb(1)Se6 octahedra, corners with four equivalent Tb(2)Se6 octahedra,  an edgeedge with one Tb(2)Se6 octahedra, and edges with two equivalent Tb(1)Se6 octahedra. The corner-sharing octahedral tilt angles range from 58-66°. There are two inequivalent Tb sites. In the first Tb site, Tb(1) is bonded to two equivalent Se(1), two equivalent Se(2), and two equivalent Se(3) atoms to form TbSe6 octahedra that share corners with four equivalent Tb(2)Se6 octahedra, corners with two equivalent Mg(1)Se4 tetrahedra, edges with two equivalent Tb(1)Se6 octahedra, edges with two equivalent Tb(2)Se6 octahedra, and edges with two equivalent Mg(1)Se4 tetrahedra. The corner-sharing octahedral tilt angles range from 50-54°. In the second Tb site, Tb(2) is bonded to one Se(2), one Se(3), and four equivalent Se(1) atoms to form TbSe6 octahedra that share corners with four equivalent Tb(1)Se6 octahedra, corners with four equivalent Tb(2)Se6 octahedra, corners with four equivalent Mg(1)Se4 tetrahedra, edges with two equivalent Tb(1)Se6 octahedra, and  an edgeedge with one Mg(1)Se4 tetrahedra. The corner-sharing octahedral tilt angles range from 46-54°. There are three inequivalent Se sites. In the first Se site, Se(1) is bonded in a rectangular see-saw-like geometry to one Mg(1), one Tb(1), and two equivalent Tb(2) atoms. In the second Se site, Se(2) is bonded in a rectangular see-saw-like geometry to one Mg(1), one Tb(2), and two equivalent Tb(1) atoms. In the third Se site, Se(3) is bonded to one Mg(1), one Tb(2), and two equivalent Tb(1) atoms to form distorted corner-sharing SeTb3Mg trigonal pyramids.
MgTb2Se4 is Spinel-like structured and crystallizes in the orthorhombic Pnma space group. Mg(1) is bonded to one Se(2), one Se(3), and two equivalent Se(1) atoms to form MgSe4 tetrahedra that share corners with two equivalent Tb(1)Se6 octahedra, corners with four equivalent Tb(2)Se6 octahedra,  an edgeedge with one Tb(2)Se6 octahedra, and edges with two equivalent Tb(1)Se6 octahedra. The corner-sharing octahedral tilt angles range from 58-66°. The Mg(1)-Se(2) bond length is 2.59 Å. The Mg(1)-Se(3) bond length is 2.52 Å. Both Mg(1)-Se(1) bond lengths are 2.58 Å. There are two inequivalent Tb sites. In the first Tb site, Tb(1) is bonded to two equivalent Se(1), two equivalent Se(2), and two equivalent Se(3) atoms to form TbSe6 octahedra that share corners with four equivalent Tb(2)Se6 octahedra, corners with two equivalent Mg(1)Se4 tetrahedra, edges with two equivalent Tb(1)Se6 octahedra, edges with two equivalent Tb(2)Se6 octahedra, and edges with two equivalent Mg(1)Se4 tetrahedra. The corner-sharing octahedral tilt angles range from 50-54°. Both Tb(1)-Se(1) bond lengths are 2.89 Å. Both Tb(1)-Se(2) bond lengths are 2.87 Å. Both Tb(1)-Se(3) bond lengths are 2.83 Å. In the second Tb site, Tb(2) is bonded to one Se(2), one Se(3), and four equivalent Se(1) atoms to form TbSe6 octahedra that share corners with four equivalent Tb(1)Se6 octahedra, corners with four equivalent Tb(2)Se6 octahedra, corners with four equivalent Mg(1)Se4 tetrahedra, edges with two equivalent Tb(1)Se6 octahedra, and  an edgeedge with one Mg(1)Se4 tetrahedra. The corner-sharing octahedral tilt angles range from 46-54°. The Tb(2)-Se(2) bond length is 2.86 Å. The Tb(2)-Se(3) bond length is 2.88 Å. There are two shorter (2.90 Å) and two longer (2.92 Å) Tb(2)-Se(1) bond lengths. There are three inequivalent Se sites. In the first Se site, Se(1) is bonded in a rectangular see-saw-like geometry to one Mg(1), one Tb(1), and two equivalent Tb(2) atoms. In the second Se site, Se(2) is bonded in a rectangular see-saw-like geometry to one Mg(1), one Tb(2), and two equivalent Tb(1) atoms. In the third Se site, Se(3) is bonded to one Mg(1), one Tb(2), and two equivalent Tb(1) atoms to form distorted corner-sharing SeTb3Mg trigonal pyramids.