MgEr4Zr4O14 crystallizes in the monoclinic P2/m space group. Mg(1) is bonded in a linear geometry to two equivalent O(2) atoms. There are three inequivalent Er sites. In the first Er site, Er(1) is bonded in a distorted linear geometry to two equivalent O(1) atoms. In the second Er site, Er(2) is bonded in a distorted hexagonal planar geometry to one O(2), one O(5), one O(6), one O(7), and two equivalent O(3) atoms. In the third Er site, Er(3) is bonded in a 6-coordinate geometry to two equivalent O(1) and four equivalent O(7) atoms. There are three inequivalent Zr sites. In the first Zr site, Zr(1) is bonded in a distorted body-centered cubic geometry to one O(2), one O(5), one O(6), one O(7), two equivalent O(1), and two equivalent O(4) atoms. In the second Zr site, Zr(2) is bonded in a body-centered cubic geometry to two equivalent O(3), two equivalent O(4), and four equivalent O(5) atoms. In the third Zr site, Zr(3) is bonded in a body-centered cubic geometry to two equivalent O(3), two equivalent O(4), and four equivalent O(6) atoms. There are seven inequivalent O sites. In the first O site, O(1) is bonded to one Er(1), one Er(3), and two equivalent Zr(1) atoms to form distorted OEr2Zr2 tetrahedra that share corners with two equivalent O(7)Er3Zr tetrahedra, corners with two equivalent O(5)ErZr3 tetrahedra, corners with two equivalent O(6)ErZr3 tetrahedra, corners with two equivalent O(4)Zr4 tetrahedra, corners with four equivalent O(1)Er2Zr2 tetrahedra,  an edgeedge with one O(4)Zr4 tetrahedra, and edges with two equivalent O(7)Er3Zr tetrahedra. In the second O site, O(2) is bonded in a distorted trigonal planar geometry to one Mg(1), one Er(2), and one Zr(1) atom. In the third O site, O(3) is bonded to two equivalent Er(2), one Zr(2), and one Zr(3) atom to form OEr2Zr2 tetrahedra that share corners with two equivalent O(7)Er3Zr tetrahedra, corners with two equivalent O(5)ErZr3 tetrahedra, corners with two equivalent O(6)ErZr3 tetrahedra, corners with two equivalent O(4)Zr4 tetrahedra, corners with four equivalent O(3)Er2Zr2 tetrahedra,  an edgeedge with one O(4)Zr4 tetrahedra, edges with two equivalent O(5)ErZr3 tetrahedra, and edges with two equivalent O(6)ErZr3 tetrahedra. In the fourth O site, O(4) is bonded to one Zr(2), one Zr(3), and two equivalent Zr(1) atoms to form OZr4 tetrahedra that share corners with two equivalent O(1)Er2Zr2 tetrahedra, corners with two equivalent O(3)Er2Zr2 tetrahedra, corners with two equivalent O(7)Er3Zr tetrahedra, corners with two equivalent O(5)ErZr3 tetrahedra, corners with two equivalent O(6)ErZr3 tetrahedra, corners with four equivalent O(4)Zr4 tetrahedra,  an edgeedge with one O(1)Er2Zr2 tetrahedra,  an edgeedge with one O(3)Er2Zr2 tetrahedra, edges with two equivalent O(5)ErZr3 tetrahedra, and edges with two equivalent O(6)ErZr3 tetrahedra. In the fifth O site, O(5) is bonded to one Er(2), one Zr(1), and two equivalent Zr(2) atoms to form OErZr3 tetrahedra that share corners with two equivalent O(1)Er2Zr2 tetrahedra, corners with two equivalent O(3)Er2Zr2 tetrahedra, corners with two equivalent O(7)Er3Zr tetrahedra, corners with two equivalent O(6)ErZr3 tetrahedra, corners with two equivalent O(4)Zr4 tetrahedra, corners with four equivalent O(5)ErZr3 tetrahedra,  an edgeedge with one O(5)ErZr3 tetrahedra, edges with two equivalent O(3)Er2Zr2 tetrahedra, and edges with two equivalent O(4)Zr4 tetrahedra. In the sixth O site, O(6) is bonded to one Er(2), one Zr(1), and two equivalent Zr(3) atoms to form OErZr3 tetrahedra that share corners with two equivalent O(1)Er2Zr2 tetrahedra, corners with two equivalent O(3)Er2Zr2 tetrahedra, corners with two equivalent O(5)ErZr3 tetrahedra, corners with two equivalent O(4)Zr4 tetrahedra, corners with four equivalent O(6)ErZr3 tetrahedra,  an edgeedge with one O(7)Er3Zr tetrahedra,  an edgeedge with one O(6)ErZr3 tetrahedra, edges with two equivalent O(3)Er2Zr2 tetrahedra, and edges with two equivalent O(4)Zr4 tetrahedra. In the seventh O site, O(7) is bonded to one Er(2), two equivalent Er(3), and one Zr(1) atom to form OEr3Zr tetrahedra that share corners with two equivalent O(1)Er2Zr2 tetrahedra, corners with two equivalent O(3)Er2Zr2 tetrahedra, corners with two equivalent O(5)ErZr3 tetrahedra, corners with two equivalent O(4)Zr4 tetrahedra, corners with four equivalent O(7)Er3Zr tetrahedra,  an edgeedge with one O(7)Er3Zr tetrahedra,  an edgeedge with one O(6)ErZr3 tetrahedra, and edges with two equivalent O(1)Er2Zr2 tetrahedra.
MgEr4Zr4O14 crystallizes in the monoclinic P2/m space group. Mg(1) is bonded in a linear geometry to two equivalent O(2) atoms. Both Mg(1)-O(2) bond lengths are 1.77 Å. There are three inequivalent Er sites. In the first Er site, Er(1) is bonded in a distorted linear geometry to two equivalent O(1) atoms. Both Er(1)-O(1) bond lengths are 2.12 Å. In the second Er site, Er(2) is bonded in a distorted hexagonal planar geometry to one O(2), one O(5), one O(6), one O(7), and two equivalent O(3) atoms. The Er(2)-O(2) bond length is 2.15 Å. The Er(2)-O(5) bond length is 2.28 Å. The Er(2)-O(6) bond length is 2.20 Å. The Er(2)-O(7) bond length is 2.25 Å. Both Er(2)-O(3) bond lengths are 2.32 Å. In the third Er site, Er(3) is bonded in a 6-coordinate geometry to two equivalent O(1) and four equivalent O(7) atoms. Both Er(3)-O(1) bond lengths are 2.20 Å. All Er(3)-O(7) bond lengths are 2.59 Å. There are three inequivalent Zr sites. In the first Zr site, Zr(1) is bonded in a distorted body-centered cubic geometry to one O(2), one O(5), one O(6), one O(7), two equivalent O(1), and two equivalent O(4) atoms. The Zr(1)-O(2) bond length is 2.21 Å. The Zr(1)-O(5) bond length is 2.14 Å. The Zr(1)-O(6) bond length is 2.08 Å. The Zr(1)-O(7) bond length is 2.28 Å. Both Zr(1)-O(1) bond lengths are 2.64 Å. Both Zr(1)-O(4) bond lengths are 2.39 Å. In the second Zr site, Zr(2) is bonded in a body-centered cubic geometry to two equivalent O(3), two equivalent O(4), and four equivalent O(5) atoms. Both Zr(2)-O(3) bond lengths are 2.19 Å. Both Zr(2)-O(4) bond lengths are 2.29 Å. All Zr(2)-O(5) bond lengths are 2.41 Å. In the third Zr site, Zr(3) is bonded in a body-centered cubic geometry to two equivalent O(3), two equivalent O(4), and four equivalent O(6) atoms. Both Zr(3)-O(3) bond lengths are 2.18 Å. Both Zr(3)-O(4) bond lengths are 2.24 Å. All Zr(3)-O(6) bond lengths are 2.41 Å. There are seven inequivalent O sites. In the first O site, O(1) is bonded to one Er(1), one Er(3), and two equivalent Zr(1) atoms to form distorted OEr2Zr2 tetrahedra that share corners with two equivalent O(7)Er3Zr tetrahedra, corners with two equivalent O(5)ErZr3 tetrahedra, corners with two equivalent O(6)ErZr3 tetrahedra, corners with two equivalent O(4)Zr4 tetrahedra, corners with four equivalent O(1)Er2Zr2 tetrahedra,  an edgeedge with one O(4)Zr4 tetrahedra, and edges with two equivalent O(7)Er3Zr tetrahedra. In the second O site, O(2) is bonded in a distorted trigonal planar geometry to one Mg(1), one Er(2), and one Zr(1) atom. In the third O site, O(3) is bonded to two equivalent Er(2), one Zr(2), and one Zr(3) atom to form OEr2Zr2 tetrahedra that share corners with two equivalent O(7)Er3Zr tetrahedra, corners with two equivalent O(5)ErZr3 tetrahedra, corners with two equivalent O(6)ErZr3 tetrahedra, corners with two equivalent O(4)Zr4 tetrahedra, corners with four equivalent O(3)Er2Zr2 tetrahedra,  an edgeedge with one O(4)Zr4 tetrahedra, edges with two equivalent O(5)ErZr3 tetrahedra, and edges with two equivalent O(6)ErZr3 tetrahedra. In the fourth O site, O(4) is bonded to one Zr(2), one Zr(3), and two equivalent Zr(1) atoms to form OZr4 tetrahedra that share corners with two equivalent O(1)Er2Zr2 tetrahedra, corners with two equivalent O(3)Er2Zr2 tetrahedra, corners with two equivalent O(7)Er3Zr tetrahedra, corners with two equivalent O(5)ErZr3 tetrahedra, corners with two equivalent O(6)ErZr3 tetrahedra, corners with four equivalent O(4)Zr4 tetrahedra,  an edgeedge with one O(1)Er2Zr2 tetrahedra,  an edgeedge with one O(3)Er2Zr2 tetrahedra, edges with two equivalent O(5)ErZr3 tetrahedra, and edges with two equivalent O(6)ErZr3 tetrahedra. In the fifth O site, O(5) is bonded to one Er(2), one Zr(1), and two equivalent Zr(2) atoms to form OErZr3 tetrahedra that share corners with two equivalent O(1)Er2Zr2 tetrahedra, corners with two equivalent O(3)Er2Zr2 tetrahedra, corners with two equivalent O(7)Er3Zr tetrahedra, corners with two equivalent O(6)ErZr3 tetrahedra, corners with two equivalent O(4)Zr4 tetrahedra, corners with four equivalent O(5)ErZr3 tetrahedra,  an edgeedge with one O(5)ErZr3 tetrahedra, edges with two equivalent O(3)Er2Zr2 tetrahedra, and edges with two equivalent O(4)Zr4 tetrahedra. In the sixth O site, O(6) is bonded to one Er(2), one Zr(1), and two equivalent Zr(3) atoms to form OErZr3 tetrahedra that share corners with two equivalent O(1)Er2Zr2 tetrahedra, corners with two equivalent O(3)Er2Zr2 tetrahedra, corners with two equivalent O(5)ErZr3 tetrahedra, corners with two equivalent O(4)Zr4 tetrahedra, corners with four equivalent O(6)ErZr3 tetrahedra,  an edgeedge with one O(7)Er3Zr tetrahedra,  an edgeedge with one O(6)ErZr3 tetrahedra, edges with two equivalent O(3)Er2Zr2 tetrahedra, and edges with two equivalent O(4)Zr4 tetrahedra. In the seventh O site, O(7) is bonded to one Er(2), two equivalent Er(3), and one Zr(1) atom to form OEr3Zr tetrahedra that share corners with two equivalent O(1)Er2Zr2 tetrahedra, corners with two equivalent O(3)Er2Zr2 tetrahedra, corners with two equivalent O(5)ErZr3 tetrahedra, corners with two equivalent O(4)Zr4 tetrahedra, corners with four equivalent O(7)Er3Zr tetrahedra,  an edgeedge with one O(7)Er3Zr tetrahedra,  an edgeedge with one O(6)ErZr3 tetrahedra, and edges with two equivalent O(1)Er2Zr2 tetrahedra.