MgLa2Se4 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 La(1)Se6 octahedra, corners with two equivalent La(2)Se6 octahedra,  an edgeedge with one La(1)Se6 octahedra, edges with three equivalent La(2)Se6 octahedra, and edges with four equivalent Mg(1)Se5 square pyramids. The corner-sharing octahedral tilt angles range from 5-6°. There are two inequivalent La sites. In the first La site, La(1) is bonded to one Se(3), two equivalent Se(4), and three equivalent Se(2) atoms to form LaSe6 octahedra that share corners with three equivalent La(2)Se6 octahedra, corners with two equivalent Mg(1)Se5 square pyramids, edges with two equivalent La(2)Se6 octahedra, edges with four equivalent La(1)Se6 octahedra, and  an edgeedge with one Mg(1)Se5 square pyramid. The corner-sharing octahedral tilt angles range from 53-60°. In the second La site, La(2) is bonded to one Se(2), one Se(4), two equivalent Se(1), and two equivalent Se(3) atoms to form LaSe6 octahedra that share corners with three equivalent La(1)Se6 octahedra, corners with two equivalent Mg(1)Se5 square pyramids, edges with two equivalent La(1)Se6 octahedra, edges with two equivalent La(2)Se6 octahedra, and edges with three equivalent Mg(1)Se5 square pyramids. The corner-sharing octahedral tilt angles range from 53-60°. There are four inequivalent Se sites. In the first Se site, Se(1) is bonded to three equivalent Mg(1) and two equivalent La(2) atoms to form SeLa2Mg3 square pyramids that share corners with two equivalent Se(4)La3Mg2 square pyramids, edges with three equivalent Se(4)La3Mg2 square pyramids, and edges with four equivalent Se(1)La2Mg3 square pyramids. In the second Se site, Se(2) is bonded in a rectangular see-saw-like geometry to one La(2) and three equivalent La(1) atoms. In the third Se site, Se(3) is bonded in a distorted T-shaped geometry to one La(1) and two equivalent La(2) atoms. In the fourth Se site, Se(4) is bonded to two equivalent Mg(1), one La(2), and two equivalent La(1) atoms to form SeLa3Mg2 square pyramids that share corners with two equivalent Se(1)La2Mg3 square pyramids, edges with two equivalent Se(4)La3Mg2 square pyramids, and edges with three equivalent Se(1)La2Mg3 square pyramids.
MgLa2Se4 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 La(1)Se6 octahedra, corners with two equivalent La(2)Se6 octahedra,  an edgeedge with one La(1)Se6 octahedra, edges with three equivalent La(2)Se6 octahedra, and edges with four equivalent Mg(1)Se5 square pyramids. The corner-sharing octahedral tilt angles range from 5-6°. Both Mg(1)-Se(4) bond lengths are 2.81 Å. There is one shorter (2.65 Å) and two longer (2.80 Å) Mg(1)-Se(1) bond lengths. There are two inequivalent La sites. In the first La site, La(1) is bonded to one Se(3), two equivalent Se(4), and three equivalent Se(2) atoms to form LaSe6 octahedra that share corners with three equivalent La(2)Se6 octahedra, corners with two equivalent Mg(1)Se5 square pyramids, edges with two equivalent La(2)Se6 octahedra, edges with four equivalent La(1)Se6 octahedra, and  an edgeedge with one Mg(1)Se5 square pyramid. The corner-sharing octahedral tilt angles range from 53-60°. The La(1)-Se(3) bond length is 2.94 Å. Both La(1)-Se(4) bond lengths are 3.06 Å. There are two shorter (2.98 Å) and one longer (3.09 Å) La(1)-Se(2) bond length. In the second La site, La(2) is bonded to one Se(2), one Se(4), two equivalent Se(1), and two equivalent Se(3) atoms to form LaSe6 octahedra that share corners with three equivalent La(1)Se6 octahedra, corners with two equivalent Mg(1)Se5 square pyramids, edges with two equivalent La(1)Se6 octahedra, edges with two equivalent La(2)Se6 octahedra, and edges with three equivalent Mg(1)Se5 square pyramids. The corner-sharing octahedral tilt angles range from 53-60°. The La(2)-Se(2) bond length is 3.03 Å. The La(2)-Se(4) bond length is 3.13 Å. Both La(2)-Se(1) bond lengths are 3.09 Å. Both La(2)-Se(3) bond lengths are 2.90 Å. There are four inequivalent Se sites. In the first Se site, Se(1) is bonded to three equivalent Mg(1) and two equivalent La(2) atoms to form SeLa2Mg3 square pyramids that share corners with two equivalent Se(4)La3Mg2 square pyramids, edges with three equivalent Se(4)La3Mg2 square pyramids, and edges with four equivalent Se(1)La2Mg3 square pyramids. In the second Se site, Se(2) is bonded in a rectangular see-saw-like geometry to one La(2) and three equivalent La(1) atoms. In the third Se site, Se(3) is bonded in a distorted T-shaped geometry to one La(1) and two equivalent La(2) atoms. In the fourth Se site, Se(4) is bonded to two equivalent Mg(1), one La(2), and two equivalent La(1) atoms to form SeLa3Mg2 square pyramids that share corners with two equivalent Se(1)La2Mg3 square pyramids, edges with two equivalent Se(4)La3Mg2 square pyramids, and edges with three equivalent Se(1)La2Mg3 square pyramids.