MgEr2Se4 crystallizes in the orthorhombic Pnma space group. Mg(1) is bonded in a trigonal pyramidal geometry to one Se(2), one Se(3), and two equivalent Se(1) atoms. There are two inequivalent Er sites. In the first Er site, Er(1) is bonded in a 7-coordinate geometry to one Se(2), two equivalent Se(3), and four equivalent Se(1) atoms. In the second Er site, Er(2) is bonded in a 6-coordinate geometry to one Se(2), one Se(3), and four equivalent Se(1) atoms. There are three inequivalent Se sites. In the first Se site, Se(1) is bonded in a 5-coordinate geometry to one Mg(1), two equivalent Er(1), and two equivalent Er(2) atoms. In the second Se site, Se(2) is bonded in a T-shaped geometry to one Mg(1), one Er(1), and one Er(2) atom. In the third Se site, Se(3) is bonded in a rectangular see-saw-like geometry to one Mg(1), one Er(2), and two equivalent Er(1) atoms.
MgEr2Se4 crystallizes in the orthorhombic Pnma space group. Mg(1) is bonded in a trigonal pyramidal geometry to one Se(2), one Se(3), and two equivalent Se(1) atoms. The Mg(1)-Se(2) bond length is 2.47 Å. The Mg(1)-Se(3) bond length is 2.61 Å. Both Mg(1)-Se(1) bond lengths are 2.58 Å. There are two inequivalent Er sites. In the first Er site, Er(1) is bonded in a 7-coordinate geometry to one Se(2), two equivalent Se(3), and four equivalent Se(1) atoms. The Er(1)-Se(2) bond length is 2.81 Å. There is one shorter (2.76 Å) and one longer (2.77 Å) Er(1)-Se(3) bond length. There are two shorter (3.03 Å) and two longer (3.34 Å) Er(1)-Se(1) bond lengths. In the second Er site, Er(2) is bonded in a 6-coordinate geometry to one Se(2), one Se(3), and four equivalent Se(1) atoms. The Er(2)-Se(2) bond length is 2.68 Å. The Er(2)-Se(3) bond length is 2.84 Å. There are two shorter (2.92 Å) and two longer (2.95 Å) Er(2)-Se(1) bond lengths. There are three inequivalent Se sites. In the first Se site, Se(1) is bonded in a 5-coordinate geometry to one Mg(1), two equivalent Er(1), and two equivalent Er(2) atoms. In the second Se site, Se(2) is bonded in a T-shaped geometry to one Mg(1), one Er(1), and one Er(2) atom. In the third Se site, Se(3) is bonded in a rectangular see-saw-like geometry to one Mg(1), one Er(2), and two equivalent Er(1) atoms.