MgTb2Se4 crystallizes in the orthorhombic Pnma space group. Mg(1) is bonded to two equivalent Se(4) and three equivalent Se(1) atoms to form MgSe5 square pyramids that share corners with two equivalent Tb(1)Se6 octahedra, corners with two equivalent Tb(2)Se6 octahedra,  an edgeedge with one Tb(1)Se6 octahedra, edges with three equivalent Tb(2)Se6 octahedra, and edges with four equivalent Mg(1)Se5 square pyramids. The corner-sharing octahedral tilt angles are 4°. There are two inequivalent Tb sites. In the first Tb site, Tb(1) is bonded to one Se(3), two equivalent Se(4), and three equivalent Se(2) atoms to form TbSe6 octahedra that share corners with three equivalent Tb(2)Se6 octahedra, corners with two equivalent Mg(1)Se5 square pyramids, edges with two equivalent Tb(2)Se6 octahedra, edges with four equivalent Tb(1)Se6 octahedra, and  an edgeedge with one Mg(1)Se5 square pyramid. The corner-sharing octahedral tilt angles range from 53-58°. In the second Tb site, Tb(2) is bonded to one Se(2), one Se(4), two equivalent Se(1), and two equivalent Se(3) atoms to form TbSe6 octahedra that share corners with three equivalent Tb(1)Se6 octahedra, corners with two equivalent Mg(1)Se5 square pyramids, edges with two equivalent Tb(1)Se6 octahedra, edges with two equivalent Tb(2)Se6 octahedra, and edges with three equivalent Mg(1)Se5 square pyramids. The corner-sharing octahedral tilt angles range from 53-58°. There are four inequivalent Se sites. In the first Se site, Se(1) is bonded to three equivalent Mg(1) and two equivalent Tb(2) atoms to form SeTb2Mg3 square pyramids that share corners with two equivalent Se(4)Tb3Mg2 square pyramids, edges with three equivalent Se(4)Tb3Mg2 square pyramids, and edges with four equivalent Se(1)Tb2Mg3 square pyramids. In the second Se site, Se(2) is bonded in a rectangular see-saw-like geometry to one Tb(2) and three equivalent Tb(1) atoms. In the third Se site, Se(3) is bonded in a distorted T-shaped geometry to one Tb(1) and two equivalent Tb(2) atoms. In the fourth Se site, Se(4) is bonded to two equivalent Mg(1), one Tb(2), and two equivalent Tb(1) atoms to form SeTb3Mg2 square pyramids that share corners with two equivalent Se(1)Tb2Mg3 square pyramids, edges with two equivalent Se(4)Tb3Mg2 square pyramids, and edges with three equivalent Se(1)Tb2Mg3 square pyramids.
MgTb2Se4 crystallizes in the orthorhombic Pnma space group. Mg(1) is bonded to two equivalent Se(4) and three equivalent Se(1) atoms to form MgSe5 square pyramids that share corners with two equivalent Tb(1)Se6 octahedra, corners with two equivalent Tb(2)Se6 octahedra,  an edgeedge with one Tb(1)Se6 octahedra, edges with three equivalent Tb(2)Se6 octahedra, and edges with four equivalent Mg(1)Se5 square pyramids. The corner-sharing octahedral tilt angles are 4°. Both Mg(1)-Se(4) bond lengths are 2.76 Å. There is one shorter (2.66 Å) and two longer (2.74 Å) Mg(1)-Se(1) bond lengths. There are two inequivalent Tb sites. In the first Tb site, Tb(1) is bonded to one Se(3), two equivalent Se(4), and three equivalent Se(2) atoms to form TbSe6 octahedra that share corners with three equivalent Tb(2)Se6 octahedra, corners with two equivalent Mg(1)Se5 square pyramids, edges with two equivalent Tb(2)Se6 octahedra, edges with four equivalent Tb(1)Se6 octahedra, and  an edgeedge with one Mg(1)Se5 square pyramid. The corner-sharing octahedral tilt angles range from 53-58°. The Tb(1)-Se(3) bond length is 2.84 Å. Both Tb(1)-Se(4) bond lengths are 2.91 Å. There are two shorter (2.86 Å) and one longer (2.96 Å) Tb(1)-Se(2) bond length. In the second Tb site, Tb(2) is bonded to one Se(2), one Se(4), two equivalent Se(1), and two equivalent Se(3) atoms to form TbSe6 octahedra that share corners with three equivalent Tb(1)Se6 octahedra, corners with two equivalent Mg(1)Se5 square pyramids, edges with two equivalent Tb(1)Se6 octahedra, edges with two equivalent Tb(2)Se6 octahedra, and edges with three equivalent Mg(1)Se5 square pyramids. The corner-sharing octahedral tilt angles range from 53-58°. The Tb(2)-Se(2) bond length is 2.95 Å. The Tb(2)-Se(4) bond length is 2.98 Å. Both Tb(2)-Se(1) bond lengths are 2.95 Å. Both Tb(2)-Se(3) bond lengths are 2.78 Å. There are four inequivalent Se sites. In the first Se site, Se(1) is bonded to three equivalent Mg(1) and two equivalent Tb(2) atoms to form SeTb2Mg3 square pyramids that share corners with two equivalent Se(4)Tb3Mg2 square pyramids, edges with three equivalent Se(4)Tb3Mg2 square pyramids, and edges with four equivalent Se(1)Tb2Mg3 square pyramids. In the second Se site, Se(2) is bonded in a rectangular see-saw-like geometry to one Tb(2) and three equivalent Tb(1) atoms. In the third Se site, Se(3) is bonded in a distorted T-shaped geometry to one Tb(1) and two equivalent Tb(2) atoms. In the fourth Se site, Se(4) is bonded to two equivalent Mg(1), one Tb(2), and two equivalent Tb(1) atoms to form SeTb3Mg2 square pyramids that share corners with two equivalent Se(1)Tb2Mg3 square pyramids, edges with two equivalent Se(4)Tb3Mg2 square pyramids, and edges with three equivalent Se(1)Tb2Mg3 square pyramids.