MgEr4Ti2O10 crystallizes in the monoclinic Pm space group. Mg(1) is bonded to one O(4) and four equivalent O(1) atoms to form distorted MgO5 square pyramids that share corners with four equivalent Er(1)O6 pentagonal pyramids, edges with two equivalent Mg(1)O5 square pyramids, and faces with two equivalent Ti(1)O6 pentagonal pyramids. There are two inequivalent Er sites. In the first Er site, Er(1) is bonded to one O(1), one O(3), one O(5), one O(6), and two equivalent O(2) atoms to form distorted ErO6 pentagonal pyramids that share  a cornercorner with one Ti(2)O6 octahedra, corners with three equivalent Er(1)O6 pentagonal pyramids, corners with two equivalent Mg(1)O5 square pyramids, edges with two equivalent Ti(2)O6 octahedra, and  an edgeedge with one Ti(1)O6 pentagonal pyramid. The corner-sharing octahedral tilt angles are 31°. In the second Er site, Er(2) is bonded in a 6-coordinate geometry to one O(1), one O(2), one O(4), one O(6), and two equivalent O(5) atoms. There are two inequivalent Ti sites. In the first Ti site, Ti(1) is bonded to two equivalent O(1), two equivalent O(4), and two equivalent O(5) atoms to form TiO6 pentagonal pyramids that share corners with two equivalent Ti(1)O6 pentagonal pyramids, edges with two equivalent Er(1)O6 pentagonal pyramids, and faces with two equivalent Mg(1)O5 square pyramids. In the second Ti site, Ti(2) is bonded to two equivalent O(2), two equivalent O(3), and two equivalent O(6) atoms to form TiO6 octahedra that share corners with two equivalent Ti(2)O6 octahedra, corners with two equivalent Er(1)O6 pentagonal pyramids, and edges with four equivalent Er(1)O6 pentagonal pyramids. The corner-sharing octahedral tilt angles are 32°. There are six inequivalent O sites. In the first O site, O(1) is bonded to two equivalent Mg(1), one Er(1), one Er(2), and one Ti(1) atom to form OEr2Mg2Ti trigonal bipyramids that share  a cornercorner with one O(3)Er2Ti2 tetrahedra, corners with three equivalent O(5)Er3Ti tetrahedra,  a cornercorner with one O(4)Er2MgTi2 trigonal bipyramid, corners with four equivalent O(1)Er2Mg2Ti trigonal bipyramids, corners with three equivalent O(2)Er3Ti trigonal pyramids,  an edgeedge with one O(5)Er3Ti tetrahedra, edges with two equivalent O(4)Er2MgTi2 trigonal bipyramids, and  a faceface with one O(1)Er2Mg2Ti trigonal bipyramid. In the second O site, O(2) is bonded to one Er(2), two equivalent Er(1), and one Ti(2) atom to form OEr3Ti trigonal pyramids that share  a cornercorner with one O(4)Er2MgTi2 trigonal bipyramid, corners with three equivalent O(1)Er2Mg2Ti trigonal bipyramids, corners with three equivalent O(2)Er3Ti trigonal pyramids, edges with two equivalent O(3)Er2Ti2 tetrahedra, and edges with two equivalent O(5)Er3Ti tetrahedra. In the third O site, O(3) is bonded to two equivalent Er(1) and two equivalent Ti(2) atoms to form distorted OEr2Ti2 tetrahedra that share corners with two equivalent O(3)Er2Ti2 tetrahedra, corners with two equivalent O(5)Er3Ti tetrahedra, corners with two equivalent O(1)Er2Mg2Ti trigonal bipyramids, and edges with four equivalent O(2)Er3Ti trigonal pyramids. In the fourth O site, O(4) is bonded to one Mg(1), two equivalent Er(2), and two equivalent Ti(1) atoms to form OEr2MgTi2 trigonal bipyramids that share corners with two equivalent O(1)Er2Mg2Ti trigonal bipyramids, corners with two equivalent O(4)Er2MgTi2 trigonal bipyramids, corners with two equivalent O(2)Er3Ti trigonal pyramids, edges with four equivalent O(5)Er3Ti tetrahedra, and edges with four equivalent O(1)Er2Mg2Ti trigonal bipyramids. In the fifth O site, O(5) is bonded to one Er(1), two equivalent Er(2), and one Ti(1) atom to form OEr3Ti tetrahedra that share  a cornercorner with one O(3)Er2Ti2 tetrahedra, corners with three equivalent O(5)Er3Ti tetrahedra, corners with three equivalent O(1)Er2Mg2Ti trigonal bipyramids,  an edgeedge with one O(1)Er2Mg2Ti trigonal bipyramid, edges with two equivalent O(4)Er2MgTi2 trigonal bipyramids, and edges with two equivalent O(2)Er3Ti trigonal pyramids. In the sixth O site, O(6) is bonded in a distorted trigonal non-coplanar geometry to one Er(1), one Er(2), and one Ti(2) atom.
MgEr4Ti2O10 crystallizes in the monoclinic Pm space group. Mg(1) is bonded to one O(4) and four equivalent O(1) atoms to form distorted MgO5 square pyramids that share corners with four equivalent Er(1)O6 pentagonal pyramids, edges with two equivalent Mg(1)O5 square pyramids, and faces with two equivalent Ti(1)O6 pentagonal pyramids. The Mg(1)-O(4) bond length is 2.05 Å. There are two shorter (2.38 Å) and two longer (2.48 Å) Mg(1)-O(1) bond lengths. There are two inequivalent Er sites. In the first Er site, Er(1) is bonded to one O(1), one O(3), one O(5), one O(6), and two equivalent O(2) atoms to form distorted ErO6 pentagonal pyramids that share  a cornercorner with one Ti(2)O6 octahedra, corners with three equivalent Er(1)O6 pentagonal pyramids, corners with two equivalent Mg(1)O5 square pyramids, edges with two equivalent Ti(2)O6 octahedra, and  an edgeedge with one Ti(1)O6 pentagonal pyramid. The corner-sharing octahedral tilt angles are 31°. The Er(1)-O(1) bond length is 2.32 Å. The Er(1)-O(3) bond length is 2.37 Å. The Er(1)-O(5) bond length is 2.26 Å. The Er(1)-O(6) bond length is 2.21 Å. Both Er(1)-O(2) bond lengths are 2.17 Å. In the second Er site, Er(2) is bonded in a 6-coordinate geometry to one O(1), one O(2), one O(4), one O(6), and two equivalent O(5) atoms. The Er(2)-O(1) bond length is 2.22 Å. The Er(2)-O(2) bond length is 2.37 Å. The Er(2)-O(4) bond length is 2.55 Å. The Er(2)-O(6) bond length is 2.20 Å. Both Er(2)-O(5) bond lengths are 2.18 Å. There are two inequivalent Ti sites. In the first Ti site, Ti(1) is bonded to two equivalent O(1), two equivalent O(4), and two equivalent O(5) atoms to form TiO6 pentagonal pyramids that share corners with two equivalent Ti(1)O6 pentagonal pyramids, edges with two equivalent Er(1)O6 pentagonal pyramids, and faces with two equivalent Mg(1)O5 square pyramids. Both Ti(1)-O(1) bond lengths are 2.22 Å. There is one shorter (2.02 Å) and one longer (2.03 Å) Ti(1)-O(4) bond length. Both Ti(1)-O(5) bond lengths are 2.04 Å. In the second Ti site, Ti(2) is bonded to two equivalent O(2), two equivalent O(3), and two equivalent O(6) atoms to form TiO6 octahedra that share corners with two equivalent Ti(2)O6 octahedra, corners with two equivalent Er(1)O6 pentagonal pyramids, and edges with four equivalent Er(1)O6 pentagonal pyramids. The corner-sharing octahedral tilt angles are 32°. Both Ti(2)-O(2) bond lengths are 2.10 Å. There is one shorter (2.10 Å) and one longer (2.12 Å) Ti(2)-O(3) bond length. Both Ti(2)-O(6) bond lengths are 1.90 Å. There are six inequivalent O sites. In the first O site, O(1) is bonded to two equivalent Mg(1), one Er(1), one Er(2), and one Ti(1) atom to form OEr2Mg2Ti trigonal bipyramids that share  a cornercorner with one O(3)Er2Ti2 tetrahedra, corners with three equivalent O(5)Er3Ti tetrahedra,  a cornercorner with one O(4)Er2MgTi2 trigonal bipyramid, corners with four equivalent O(1)Er2Mg2Ti trigonal bipyramids, corners with three equivalent O(2)Er3Ti trigonal pyramids,  an edgeedge with one O(5)Er3Ti tetrahedra, edges with two equivalent O(4)Er2MgTi2 trigonal bipyramids, and  a faceface with one O(1)Er2Mg2Ti trigonal bipyramid. In the second O site, O(2) is bonded to one Er(2), two equivalent Er(1), and one Ti(2) atom to form OEr3Ti trigonal pyramids that share  a cornercorner with one O(4)Er2MgTi2 trigonal bipyramid, corners with three equivalent O(1)Er2Mg2Ti trigonal bipyramids, corners with three equivalent O(2)Er3Ti trigonal pyramids, edges with two equivalent O(3)Er2Ti2 tetrahedra, and edges with two equivalent O(5)Er3Ti tetrahedra. In the third O site, O(3) is bonded to two equivalent Er(1) and two equivalent Ti(2) atoms to form distorted OEr2Ti2 tetrahedra that share corners with two equivalent O(3)Er2Ti2 tetrahedra, corners with two equivalent O(5)Er3Ti tetrahedra, corners with two equivalent O(1)Er2Mg2Ti trigonal bipyramids, and edges with four equivalent O(2)Er3Ti trigonal pyramids. In the fourth O site, O(4) is bonded to one Mg(1), two equivalent Er(2), and two equivalent Ti(1) atoms to form OEr2MgTi2 trigonal bipyramids that share corners with two equivalent O(1)Er2Mg2Ti trigonal bipyramids, corners with two equivalent O(4)Er2MgTi2 trigonal bipyramids, corners with two equivalent O(2)Er3Ti trigonal pyramids, edges with four equivalent O(5)Er3Ti tetrahedra, and edges with four equivalent O(1)Er2Mg2Ti trigonal bipyramids. In the fifth O site, O(5) is bonded to one Er(1), two equivalent Er(2), and one Ti(1) atom to form OEr3Ti tetrahedra that share  a cornercorner with one O(3)Er2Ti2 tetrahedra, corners with three equivalent O(5)Er3Ti tetrahedra, corners with three equivalent O(1)Er2Mg2Ti trigonal bipyramids,  an edgeedge with one O(1)Er2Mg2Ti trigonal bipyramid, edges with two equivalent O(4)Er2MgTi2 trigonal bipyramids, and edges with two equivalent O(2)Er3Ti trigonal pyramids. In the sixth O site, O(6) is bonded in a distorted trigonal non-coplanar geometry to one Er(1), one Er(2), and one Ti(2) atom.