MgEr2Cu(GeO3)4 crystallizes in the triclinic P-1 space group. Mg(1) is bonded to two equivalent O(4), two equivalent O(5), and two equivalent O(6) atoms to form distorted MgO6 octahedra that share corners with four equivalent Er(1)O7 pentagonal bipyramids, edges with two equivalent Ge(1)O4 tetrahedra, and edges with two equivalent Ge(2)O4 tetrahedra. Er(1) is bonded to one O(3), one O(5), one O(6), two equivalent O(1), and two equivalent O(2) atoms to form distorted ErO7 pentagonal bipyramids that share corners with two equivalent Mg(1)O6 octahedra, corners with two equivalent Ge(2)O4 tetrahedra, corners with four equivalent Ge(1)O4 tetrahedra, edges with two equivalent Er(1)O7 pentagonal bipyramids, and  an edgeedge with one Ge(2)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 39-41°. Cu(1) is bonded in a square co-planar geometry to two equivalent O(5) and two equivalent O(6) atoms. There are two inequivalent Ge sites. In the first Ge site, Ge(1) is bonded to one O(1), one O(3), one O(4), and one O(6) atom to form GeO4 tetrahedra that share corners with four equivalent Er(1)O7 pentagonal bipyramids, corners with two equivalent Ge(2)O4 tetrahedra, and  an edgeedge with one Mg(1)O6 octahedra. In the second Ge site, Ge(2) is bonded to one O(2), one O(3), one O(4), and one O(5) atom to form GeO4 tetrahedra that share corners with two equivalent Er(1)O7 pentagonal bipyramids, corners with two equivalent Ge(1)O4 tetrahedra,  an edgeedge with one Mg(1)O6 octahedra, and  an edgeedge with one Er(1)O7 pentagonal bipyramid. There are six inequivalent O sites. In the first O site, O(1) is bonded in a distorted trigonal planar geometry to two equivalent Er(1) and one Ge(1) atom. In the second O site, O(2) is bonded in a distorted trigonal planar geometry to two equivalent Er(1) and one Ge(2) atom. In the third O site, O(3) is bonded in a 3-coordinate geometry to one Er(1), one Ge(1), and one Ge(2) atom. In the fourth O site, O(4) is bonded in a T-shaped geometry to one Mg(1), one Ge(1), and one Ge(2) atom. In the fifth O site, O(5) is bonded in a distorted rectangular see-saw-like geometry to one Mg(1), one Er(1), one Cu(1), and one Ge(2) atom. In the sixth O site, O(6) is bonded in a 4-coordinate geometry to one Mg(1), one Er(1), one Cu(1), and one Ge(1) atom.
MgEr2Cu(GeO3)4 crystallizes in the triclinic P-1 space group. Mg(1) is bonded to two equivalent O(4), two equivalent O(5), and two equivalent O(6) atoms to form distorted MgO6 octahedra that share corners with four equivalent Er(1)O7 pentagonal bipyramids, edges with two equivalent Ge(1)O4 tetrahedra, and edges with two equivalent Ge(2)O4 tetrahedra. Both Mg(1)-O(4) bond lengths are 2.15 Å. Both Mg(1)-O(5) bond lengths are 2.16 Å. Both Mg(1)-O(6) bond lengths are 2.08 Å. Er(1) is bonded to one O(3), one O(5), one O(6), two equivalent O(1), and two equivalent O(2) atoms to form distorted ErO7 pentagonal bipyramids that share corners with two equivalent Mg(1)O6 octahedra, corners with two equivalent Ge(2)O4 tetrahedra, corners with four equivalent Ge(1)O4 tetrahedra, edges with two equivalent Er(1)O7 pentagonal bipyramids, and  an edgeedge with one Ge(2)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 39-41°. The Er(1)-O(3) bond length is 2.70 Å. The Er(1)-O(5) bond length is 2.27 Å. The Er(1)-O(6) bond length is 2.28 Å. There is one shorter (2.26 Å) and one longer (2.49 Å) Er(1)-O(1) bond length. There is one shorter (2.28 Å) and one longer (2.34 Å) Er(1)-O(2) bond length. Cu(1) is bonded in a square co-planar geometry to two equivalent O(5) and two equivalent O(6) atoms. Both Cu(1)-O(5) bond lengths are 2.14 Å. Both Cu(1)-O(6) bond lengths are 2.23 Å. There are two inequivalent Ge sites. In the first Ge site, Ge(1) is bonded to one O(1), one O(3), one O(4), and one O(6) atom to form GeO4 tetrahedra that share corners with four equivalent Er(1)O7 pentagonal bipyramids, corners with two equivalent Ge(2)O4 tetrahedra, and  an edgeedge with one Mg(1)O6 octahedra. The Ge(1)-O(1) bond length is 1.76 Å. The Ge(1)-O(3) bond length is 1.80 Å. The Ge(1)-O(4) bond length is 1.86 Å. The Ge(1)-O(6) bond length is 1.80 Å. In the second Ge site, Ge(2) is bonded to one O(2), one O(3), one O(4), and one O(5) atom to form GeO4 tetrahedra that share corners with two equivalent Er(1)O7 pentagonal bipyramids, corners with two equivalent Ge(1)O4 tetrahedra,  an edgeedge with one Mg(1)O6 octahedra, and  an edgeedge with one Er(1)O7 pentagonal bipyramid. The Ge(2)-O(2) bond length is 1.76 Å. The Ge(2)-O(3) bond length is 1.82 Å. The Ge(2)-O(4) bond length is 1.86 Å. The Ge(2)-O(5) bond length is 1.82 Å. There are six inequivalent O sites. In the first O site, O(1) is bonded in a distorted trigonal planar geometry to two equivalent Er(1) and one Ge(1) atom. In the second O site, O(2) is bonded in a distorted trigonal planar geometry to two equivalent Er(1) and one Ge(2) atom. In the third O site, O(3) is bonded in a 3-coordinate geometry to one Er(1), one Ge(1), and one Ge(2) atom. In the fourth O site, O(4) is bonded in a T-shaped geometry to one Mg(1), one Ge(1), and one Ge(2) atom. In the fifth O site, O(5) is bonded in a distorted rectangular see-saw-like geometry to one Mg(1), one Er(1), one Cu(1), and one Ge(2) atom. In the sixth O site, O(6) is bonded in a 4-coordinate geometry to one Mg(1), one Er(1), one Cu(1), and one Ge(1) atom.