MgPm2Se4 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 Pm(1)Se6 octahedra, corners with two equivalent Pm(2)Se6 octahedra,  an edgeedge with one Pm(1)Se6 octahedra, edges with three equivalent Pm(2)Se6 octahedra, and edges with four equivalent Mg(1)Se5 square pyramids. The corner-sharing octahedral tilt angles range from 4-6°. There are two inequivalent Pm sites. In the first Pm site, Pm(1) is bonded to one Se(3), two equivalent Se(4), and three equivalent Se(2) atoms to form PmSe6 octahedra that share corners with three equivalent Pm(2)Se6 octahedra, corners with two equivalent Mg(1)Se5 square pyramids, edges with two equivalent Pm(2)Se6 octahedra, edges with four equivalent Pm(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 Pm site, Pm(2) is bonded to one Se(2), one Se(4), two equivalent Se(1), and two equivalent Se(3) atoms to form PmSe6 octahedra that share corners with three equivalent Pm(1)Se6 octahedra, corners with two equivalent Mg(1)Se5 square pyramids, edges with two equivalent Pm(1)Se6 octahedra, edges with two equivalent Pm(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 Pm(2) atoms to form SePm2Mg3 square pyramids that share corners with two equivalent Se(4)Pm3Mg2 square pyramids, edges with three equivalent Se(4)Pm3Mg2 square pyramids, and edges with four equivalent Se(1)Pm2Mg3 square pyramids. In the second Se site, Se(2) is bonded in a rectangular see-saw-like geometry to one Pm(2) and three equivalent Pm(1) atoms. In the third Se site, Se(3) is bonded in a distorted T-shaped geometry to one Pm(1) and two equivalent Pm(2) atoms. In the fourth Se site, Se(4) is bonded to two equivalent Mg(1), one Pm(2), and two equivalent Pm(1) atoms to form SePm3Mg2 square pyramids that share corners with two equivalent Se(1)Pm2Mg3 square pyramids, edges with two equivalent Se(4)Pm3Mg2 square pyramids, and edges with three equivalent Se(1)Pm2Mg3 square pyramids.
MgPm2Se4 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 Pm(1)Se6 octahedra, corners with two equivalent Pm(2)Se6 octahedra,  an edgeedge with one Pm(1)Se6 octahedra, edges with three equivalent Pm(2)Se6 octahedra, and edges with four equivalent Mg(1)Se5 square pyramids. The corner-sharing octahedral tilt angles range from 4-6°. Both Mg(1)-Se(4) bond lengths are 2.78 Å. There is one shorter (2.67 Å) and two longer (2.76 Å) Mg(1)-Se(1) bond lengths. There are two inequivalent Pm sites. In the first Pm site, Pm(1) is bonded to one Se(3), two equivalent Se(4), and three equivalent Se(2) atoms to form PmSe6 octahedra that share corners with three equivalent Pm(2)Se6 octahedra, corners with two equivalent Mg(1)Se5 square pyramids, edges with two equivalent Pm(2)Se6 octahedra, edges with four equivalent Pm(1)Se6 octahedra, and  an edgeedge with one Mg(1)Se5 square pyramid. The corner-sharing octahedral tilt angles range from 53-59°. The Pm(1)-Se(3) bond length is 2.89 Å. Both Pm(1)-Se(4) bond lengths are 2.97 Å. There are two shorter (2.91 Å) and one longer (3.02 Å) Pm(1)-Se(2) bond length. In the second Pm site, Pm(2) is bonded to one Se(2), one Se(4), two equivalent Se(1), and two equivalent Se(3) atoms to form PmSe6 octahedra that share corners with three equivalent Pm(1)Se6 octahedra, corners with two equivalent Mg(1)Se5 square pyramids, edges with two equivalent Pm(1)Se6 octahedra, edges with two equivalent Pm(2)Se6 octahedra, and edges with three equivalent Mg(1)Se5 square pyramids. The corner-sharing octahedral tilt angles range from 53-59°. The Pm(2)-Se(2) bond length is 2.98 Å. The Pm(2)-Se(4) bond length is 3.05 Å. Both Pm(2)-Se(1) bond lengths are 3.03 Å. Both Pm(2)-Se(3) bond lengths are 2.83 Å. There are four inequivalent Se sites. In the first Se site, Se(1) is bonded to three equivalent Mg(1) and two equivalent Pm(2) atoms to form SePm2Mg3 square pyramids that share corners with two equivalent Se(4)Pm3Mg2 square pyramids, edges with three equivalent Se(4)Pm3Mg2 square pyramids, and edges with four equivalent Se(1)Pm2Mg3 square pyramids. In the second Se site, Se(2) is bonded in a rectangular see-saw-like geometry to one Pm(2) and three equivalent Pm(1) atoms. In the third Se site, Se(3) is bonded in a distorted T-shaped geometry to one Pm(1) and two equivalent Pm(2) atoms. In the fourth Se site, Se(4) is bonded to two equivalent Mg(1), one Pm(2), and two equivalent Pm(1) atoms to form SePm3Mg2 square pyramids that share corners with two equivalent Se(1)Pm2Mg3 square pyramids, edges with two equivalent Se(4)Pm3Mg2 square pyramids, and edges with three equivalent Se(1)Pm2Mg3 square pyramids.