MgCe2Se4 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 Ce(1)Se6 octahedra, corners with two equivalent Ce(2)Se6 octahedra,  an edgeedge with one Ce(1)Se6 octahedra, edges with three equivalent Ce(2)Se6 octahedra, and edges with four equivalent Mg(1)Se5 square pyramids. The corner-sharing octahedral tilt angles are 5°. There are two inequivalent Ce sites. In the first Ce site, Ce(1) is bonded to one Se(3), two equivalent Se(4), and three equivalent Se(2) atoms to form CeSe6 octahedra that share corners with three equivalent Ce(2)Se6 octahedra, corners with two equivalent Mg(1)Se5 square pyramids, edges with two equivalent Ce(2)Se6 octahedra, edges with four equivalent Ce(1)Se6 octahedra, and  an edgeedge with one Mg(1)Se5 square pyramid. The corner-sharing octahedral tilt angles range from 53-59°. In the second Ce site, Ce(2) is bonded to one Se(2), one Se(4), two equivalent Se(1), and two equivalent Se(3) atoms to form CeSe6 octahedra that share corners with three equivalent Ce(1)Se6 octahedra, corners with two equivalent Mg(1)Se5 square pyramids, edges with two equivalent Ce(1)Se6 octahedra, edges with two equivalent Ce(2)Se6 octahedra, and edges with three equivalent Mg(1)Se5 square pyramids. The corner-sharing octahedral tilt angles range from 53-59°. There are four inequivalent Se sites. In the first Se site, Se(1) is bonded to three equivalent Mg(1) and two equivalent Ce(2) atoms to form SeCe2Mg3 square pyramids that share corners with two equivalent Se(4)Ce3Mg2 square pyramids, edges with three equivalent Se(4)Ce3Mg2 square pyramids, and edges with four equivalent Se(1)Ce2Mg3 square pyramids. In the second Se site, Se(2) is bonded in a rectangular see-saw-like geometry to one Ce(2) and three equivalent Ce(1) atoms. In the third Se site, Se(3) is bonded in a distorted T-shaped geometry to one Ce(1) and two equivalent Ce(2) atoms. In the fourth Se site, Se(4) is bonded to two equivalent Mg(1), one Ce(2), and two equivalent Ce(1) atoms to form SeCe3Mg2 square pyramids that share corners with two equivalent Se(1)Ce2Mg3 square pyramids, edges with two equivalent Se(4)Ce3Mg2 square pyramids, and edges with three equivalent Se(1)Ce2Mg3 square pyramids.
MgCe2Se4 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 Ce(1)Se6 octahedra, corners with two equivalent Ce(2)Se6 octahedra,  an edgeedge with one Ce(1)Se6 octahedra, edges with three equivalent Ce(2)Se6 octahedra, and edges with four equivalent Mg(1)Se5 square pyramids. The corner-sharing octahedral tilt angles are 5°. Both Mg(1)-Se(4) bond lengths are 2.77 Å. There is one shorter (2.64 Å) and two longer (2.77 Å) Mg(1)-Se(1) bond lengths. There are two inequivalent Ce sites. In the first Ce site, Ce(1) is bonded to one Se(3), two equivalent Se(4), and three equivalent Se(2) atoms to form CeSe6 octahedra that share corners with three equivalent Ce(2)Se6 octahedra, corners with two equivalent Mg(1)Se5 square pyramids, edges with two equivalent Ce(2)Se6 octahedra, edges with four equivalent Ce(1)Se6 octahedra, and  an edgeedge with one Mg(1)Se5 square pyramid. The corner-sharing octahedral tilt angles range from 53-59°. The Ce(1)-Se(3) bond length is 2.91 Å. Both Ce(1)-Se(4) bond lengths are 3.01 Å. There are two shorter (2.94 Å) and one longer (2.99 Å) Ce(1)-Se(2) bond length. In the second Ce site, Ce(2) is bonded to one Se(2), one Se(4), two equivalent Se(1), and two equivalent Se(3) atoms to form CeSe6 octahedra that share corners with three equivalent Ce(1)Se6 octahedra, corners with two equivalent Mg(1)Se5 square pyramids, edges with two equivalent Ce(1)Se6 octahedra, edges with two equivalent Ce(2)Se6 octahedra, and edges with three equivalent Mg(1)Se5 square pyramids. The corner-sharing octahedral tilt angles range from 53-59°. The Ce(2)-Se(2) bond length is 2.98 Å. The Ce(2)-Se(4) bond length is 3.06 Å. Both Ce(2)-Se(1) bond lengths are 3.04 Å. Both Ce(2)-Se(3) bond lengths are 2.86 Å. There are four inequivalent Se sites. In the first Se site, Se(1) is bonded to three equivalent Mg(1) and two equivalent Ce(2) atoms to form SeCe2Mg3 square pyramids that share corners with two equivalent Se(4)Ce3Mg2 square pyramids, edges with three equivalent Se(4)Ce3Mg2 square pyramids, and edges with four equivalent Se(1)Ce2Mg3 square pyramids. In the second Se site, Se(2) is bonded in a rectangular see-saw-like geometry to one Ce(2) and three equivalent Ce(1) atoms. In the third Se site, Se(3) is bonded in a distorted T-shaped geometry to one Ce(1) and two equivalent Ce(2) atoms. In the fourth Se site, Se(4) is bonded to two equivalent Mg(1), one Ce(2), and two equivalent Ce(1) atoms to form SeCe3Mg2 square pyramids that share corners with two equivalent Se(1)Ce2Mg3 square pyramids, edges with two equivalent Se(4)Ce3Mg2 square pyramids, and edges with three equivalent Se(1)Ce2Mg3 square pyramids.