MgNd2Se4 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 Nd(1)Se6 octahedra, corners with two equivalent Nd(2)Se6 octahedra,  an edgeedge with one Nd(1)Se6 octahedra, edges with three equivalent Nd(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 Nd sites. In the first Nd site, Nd(1) is bonded to one Se(3), two equivalent Se(4), and three equivalent Se(2) atoms to form NdSe6 octahedra that share corners with three equivalent Nd(2)Se6 octahedra, corners with two equivalent Mg(1)Se5 square pyramids, edges with two equivalent Nd(2)Se6 octahedra, edges with four equivalent Nd(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 Nd site, Nd(2) is bonded to one Se(2), one Se(4), two equivalent Se(1), and two equivalent Se(3) atoms to form NdSe6 octahedra that share corners with three equivalent Nd(1)Se6 octahedra, corners with two equivalent Mg(1)Se5 square pyramids, edges with two equivalent Nd(1)Se6 octahedra, edges with two equivalent Nd(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 Nd(2) atoms to form SeNd2Mg3 square pyramids that share corners with two equivalent Se(4)Nd3Mg2 square pyramids, edges with three equivalent Se(4)Nd3Mg2 square pyramids, and edges with four equivalent Se(1)Nd2Mg3 square pyramids. In the second Se site, Se(2) is bonded in a rectangular see-saw-like geometry to one Nd(2) and three equivalent Nd(1) atoms. In the third Se site, Se(3) is bonded in a distorted T-shaped geometry to one Nd(1) and two equivalent Nd(2) atoms. In the fourth Se site, Se(4) is bonded to two equivalent Mg(1), one Nd(2), and two equivalent Nd(1) atoms to form SeNd3Mg2 square pyramids that share corners with two equivalent Se(1)Nd2Mg3 square pyramids, edges with two equivalent Se(4)Nd3Mg2 square pyramids, and edges with three equivalent Se(1)Nd2Mg3 square pyramids.
MgNd2Se4 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 Nd(1)Se6 octahedra, corners with two equivalent Nd(2)Se6 octahedra,  an edgeedge with one Nd(1)Se6 octahedra, edges with three equivalent Nd(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 2.79 Å. There is one shorter (2.66 Å) and two longer (2.77 Å) Mg(1)-Se(1) bond lengths. There are two inequivalent Nd sites. In the first Nd site, Nd(1) is bonded to one Se(3), two equivalent Se(4), and three equivalent Se(2) atoms to form NdSe6 octahedra that share corners with three equivalent Nd(2)Se6 octahedra, corners with two equivalent Mg(1)Se5 square pyramids, edges with two equivalent Nd(2)Se6 octahedra, edges with four equivalent Nd(1)Se6 octahedra, and  an edgeedge with one Mg(1)Se5 square pyramid. The corner-sharing octahedral tilt angles range from 53-59°. The Nd(1)-Se(3) bond length is 2.91 Å. Both Nd(1)-Se(4) bond lengths are 3.00 Å. There are two shorter (2.94 Å) and one longer (3.05 Å) Nd(1)-Se(2) bond length. In the second Nd site, Nd(2) is bonded to one Se(2), one Se(4), two equivalent Se(1), and two equivalent Se(3) atoms to form NdSe6 octahedra that share corners with three equivalent Nd(1)Se6 octahedra, corners with two equivalent Mg(1)Se5 square pyramids, edges with two equivalent Nd(1)Se6 octahedra, edges with two equivalent Nd(2)Se6 octahedra, and edges with three equivalent Mg(1)Se5 square pyramids. The corner-sharing octahedral tilt angles range from 53-59°. The Nd(2)-Se(2) bond length is 3.00 Å. The Nd(2)-Se(4) bond length is 3.09 Å. Both Nd(2)-Se(1) bond lengths are 3.05 Å. Both Nd(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 Nd(2) atoms to form SeNd2Mg3 square pyramids that share corners with two equivalent Se(4)Nd3Mg2 square pyramids, edges with three equivalent Se(4)Nd3Mg2 square pyramids, and edges with four equivalent Se(1)Nd2Mg3 square pyramids. In the second Se site, Se(2) is bonded in a rectangular see-saw-like geometry to one Nd(2) and three equivalent Nd(1) atoms. In the third Se site, Se(3) is bonded in a distorted T-shaped geometry to one Nd(1) and two equivalent Nd(2) atoms. In the fourth Se site, Se(4) is bonded to two equivalent Mg(1), one Nd(2), and two equivalent Nd(1) atoms to form SeNd3Mg2 square pyramids that share corners with two equivalent Se(1)Nd2Mg3 square pyramids, edges with two equivalent Se(4)Nd3Mg2 square pyramids, and edges with three equivalent Se(1)Nd2Mg3 square pyramids.