MgEr2V2O8 crystallizes in the monoclinic C2 space group. Mg(1) is bonded to two equivalent O(1), two equivalent O(2), and two equivalent O(4) atoms to form distorted MgO6 octahedra that share corners with two equivalent V(1)O4 tetrahedra and edges with two equivalent V(1)O4 tetrahedra. Er(1) is bonded in a 7-coordinate geometry to one O(1), two equivalent O(2), two equivalent O(3), and two equivalent O(4) atoms. V(1) is bonded to one O(1), one O(2), one O(3), and one O(4) atom to form VO4 tetrahedra that share  a cornercorner with one Mg(1)O6 octahedra and  an edgeedge with one Mg(1)O6 octahedra. The corner-sharing octahedral tilt angles are 61°. There are four inequivalent O sites. In the first O site, O(1) is bonded in a 3-coordinate geometry to one Mg(1), one Er(1), and one V(1) atom. In the second O site, O(2) is bonded to one Mg(1), two equivalent Er(1), and one V(1) atom to form distorted corner-sharing OEr2MgV trigonal pyramids. In the third O site, O(3) is bonded in a 3-coordinate geometry to two equivalent Er(1) and one V(1) atom. In the fourth O site, O(4) is bonded in a distorted rectangular see-saw-like geometry to one Mg(1), two equivalent Er(1), and one V(1) atom.
MgEr2V2O8 crystallizes in the monoclinic C2 space group. Mg(1) is bonded to two equivalent O(1), two equivalent O(2), and two equivalent O(4) atoms to form distorted MgO6 octahedra that share corners with two equivalent V(1)O4 tetrahedra and edges with two equivalent V(1)O4 tetrahedra. Both Mg(1)-O(1) bond lengths are 2.05 Å. Both Mg(1)-O(2) bond lengths are 1.96 Å. Both Mg(1)-O(4) bond lengths are 2.43 Å. Er(1) is bonded in a 7-coordinate geometry to one O(1), two equivalent O(2), two equivalent O(3), and two equivalent O(4) atoms. The Er(1)-O(1) bond length is 2.37 Å. There is one shorter (2.38 Å) and one longer (2.39 Å) Er(1)-O(2) bond length. There is one shorter (2.25 Å) and one longer (2.35 Å) Er(1)-O(3) bond length. There is one shorter (2.26 Å) and one longer (2.41 Å) Er(1)-O(4) bond length. V(1) is bonded to one O(1), one O(2), one O(3), and one O(4) atom to form VO4 tetrahedra that share  a cornercorner with one Mg(1)O6 octahedra and  an edgeedge with one Mg(1)O6 octahedra. The corner-sharing octahedral tilt angles are 61°. The V(1)-O(1) bond length is 1.77 Å. The V(1)-O(2) bond length is 1.89 Å. The V(1)-O(3) bond length is 1.76 Å. The V(1)-O(4) bond length is 1.90 Å. There are four inequivalent O sites. In the first O site, O(1) is bonded in a 3-coordinate geometry to one Mg(1), one Er(1), and one V(1) atom. In the second O site, O(2) is bonded to one Mg(1), two equivalent Er(1), and one V(1) atom to form distorted corner-sharing OEr2MgV trigonal pyramids. In the third O site, O(3) is bonded in a 3-coordinate geometry to two equivalent Er(1) and one V(1) atom. In the fourth O site, O(4) is bonded in a distorted rectangular see-saw-like geometry to one Mg(1), two equivalent Er(1), and one V(1) atom.