MgEr6Ta2O14 crystallizes in the orthorhombic Amm2 space group. Mg(1) is bonded to two equivalent O(2) and four equivalent O(4) atoms to form distorted MgO6 pentagonal pyramids that share corners with two equivalent Er(1)O7 pentagonal bipyramids, corners with four equivalent Er(3)O7 pentagonal bipyramids, edges with two equivalent Ta(1)O6 octahedra, and faces with two equivalent Er(1)O7 pentagonal bipyramids. There are three inequivalent Er sites. In the first Er site, Er(1) is bonded to one O(5), two equivalent O(2), two equivalent O(3), and two equivalent O(4) atoms to form ErO7 pentagonal bipyramids that share corners with two equivalent Ta(1)O6 octahedra, corners with three equivalent Er(1)O7 pentagonal bipyramids,  a cornercorner with one Mg(1)O6 pentagonal pyramid, edges with two equivalent Ta(1)O6 octahedra, edges with two equivalent Er(3)O7 pentagonal bipyramids, and  a faceface with one Mg(1)O6 pentagonal pyramid. The corner-sharing octahedral tilt angles are 34°. In the second Er site, Er(2) is bonded in a 6-coordinate geometry to two equivalent O(1), two equivalent O(2), and two equivalent O(3) atoms. In the third Er site, Er(3) is bonded to one O(6), two equivalent O(1), two equivalent O(3), and two equivalent O(4) atoms to form ErO7 pentagonal bipyramids that share corners with two equivalent Ta(1)O6 octahedra, corners with three equivalent Er(3)O7 pentagonal bipyramids, corners with two equivalent Mg(1)O6 pentagonal pyramids, edges with two equivalent Ta(1)O6 octahedra, and edges with two equivalent Er(1)O7 pentagonal bipyramids. The corner-sharing octahedral tilt angles are 44°. Ta(1) is bonded to one O(5), one O(6), two equivalent O(3), and two equivalent O(4) atoms to form TaO6 octahedra that share corners with two equivalent Ta(1)O6 octahedra, corners with two equivalent Er(1)O7 pentagonal bipyramids, corners with two equivalent Er(3)O7 pentagonal bipyramids, edges with two equivalent Er(1)O7 pentagonal bipyramids, edges with two equivalent Er(3)O7 pentagonal bipyramids, and  an edgeedge with one Mg(1)O6 pentagonal pyramid. The corner-sharing octahedral tilt angles range from 39-45°. There are six inequivalent O sites. In the first O site, O(1) is bonded to two equivalent Er(2) and two equivalent Er(3) atoms to form OEr4 tetrahedra that share corners with two equivalent O(1)Er4 tetrahedra, corners with four equivalent O(3)Er3Ta tetrahedra, corners with four equivalent O(2)Er4Mg trigonal bipyramids,  an edgeedge with one O(1)Er4 tetrahedra, and edges with two equivalent O(3)Er3Ta tetrahedra. In the second O site, O(2) is bonded to one Mg(1), two equivalent Er(1), and two equivalent Er(2) atoms to form distorted OEr4Mg trigonal bipyramids that share corners with four equivalent O(3)Er3Ta tetrahedra, corners with four equivalent O(1)Er4 tetrahedra, corners with two equivalent O(2)Er4Mg trigonal bipyramids, edges with two equivalent O(3)Er3Ta tetrahedra, and  a faceface with one O(2)Er4Mg trigonal bipyramid. In the third O site, O(3) is bonded to one Er(1), one Er(2), one Er(3), and one Ta(1) atom to form distorted OEr3Ta tetrahedra that share corners with two equivalent O(3)Er3Ta tetrahedra, corners with two equivalent O(1)Er4 tetrahedra, corners with two equivalent O(2)Er4Mg trigonal bipyramids,  an edgeedge with one O(3)Er3Ta tetrahedra,  an edgeedge with one O(1)Er4 tetrahedra, and  an edgeedge with one O(2)Er4Mg trigonal bipyramid. In the fourth O site, O(4) is bonded in a 4-coordinate geometry to one Mg(1), one Er(1), one Er(3), and one Ta(1) atom. In the fifth O site, O(5) is bonded in a 4-coordinate geometry to two equivalent Er(1) and two equivalent Ta(1) atoms. In the sixth O site, O(6) is bonded in a 4-coordinate geometry to two equivalent Er(3) and two equivalent Ta(1) atoms.
MgEr6Ta2O14 crystallizes in the orthorhombic Amm2 space group. Mg(1) is bonded to two equivalent O(2) and four equivalent O(4) atoms to form distorted MgO6 pentagonal pyramids that share corners with two equivalent Er(1)O7 pentagonal bipyramids, corners with four equivalent Er(3)O7 pentagonal bipyramids, edges with two equivalent Ta(1)O6 octahedra, and faces with two equivalent Er(1)O7 pentagonal bipyramids. Both Mg(1)-O(2) bond lengths are 2.21 Å. All Mg(1)-O(4) bond lengths are 2.10 Å. There are three inequivalent Er sites. In the first Er site, Er(1) is bonded to one O(5), two equivalent O(2), two equivalent O(3), and two equivalent O(4) atoms to form ErO7 pentagonal bipyramids that share corners with two equivalent Ta(1)O6 octahedra, corners with three equivalent Er(1)O7 pentagonal bipyramids,  a cornercorner with one Mg(1)O6 pentagonal pyramid, edges with two equivalent Ta(1)O6 octahedra, edges with two equivalent Er(3)O7 pentagonal bipyramids, and  a faceface with one Mg(1)O6 pentagonal pyramid. The corner-sharing octahedral tilt angles are 34°. The Er(1)-O(5) bond length is 2.47 Å. There is one shorter (2.28 Å) and one longer (2.38 Å) Er(1)-O(2) bond length. Both Er(1)-O(3) bond lengths are 2.36 Å. Both Er(1)-O(4) bond lengths are 2.27 Å. In the second Er site, Er(2) is bonded in a 6-coordinate geometry to two equivalent O(1), two equivalent O(2), and two equivalent O(3) atoms. Both Er(2)-O(1) bond lengths are 2.29 Å. Both Er(2)-O(2) bond lengths are 2.46 Å. Both Er(2)-O(3) bond lengths are 2.25 Å. In the third Er site, Er(3) is bonded to one O(6), two equivalent O(1), two equivalent O(3), and two equivalent O(4) atoms to form ErO7 pentagonal bipyramids that share corners with two equivalent Ta(1)O6 octahedra, corners with three equivalent Er(3)O7 pentagonal bipyramids, corners with two equivalent Mg(1)O6 pentagonal pyramids, edges with two equivalent Ta(1)O6 octahedra, and edges with two equivalent Er(1)O7 pentagonal bipyramids. The corner-sharing octahedral tilt angles are 44°. The Er(3)-O(6) bond length is 2.48 Å. There is one shorter (2.21 Å) and one longer (2.34 Å) Er(3)-O(1) bond length. Both Er(3)-O(3) bond lengths are 2.29 Å. Both Er(3)-O(4) bond lengths are 2.47 Å. Ta(1) is bonded to one O(5), one O(6), two equivalent O(3), and two equivalent O(4) atoms to form TaO6 octahedra that share corners with two equivalent Ta(1)O6 octahedra, corners with two equivalent Er(1)O7 pentagonal bipyramids, corners with two equivalent Er(3)O7 pentagonal bipyramids, edges with two equivalent Er(1)O7 pentagonal bipyramids, edges with two equivalent Er(3)O7 pentagonal bipyramids, and  an edgeedge with one Mg(1)O6 pentagonal pyramid. The corner-sharing octahedral tilt angles range from 39-45°. The Ta(1)-O(5) bond length is 2.02 Å. The Ta(1)-O(6) bond length is 1.98 Å. Both Ta(1)-O(3) bond lengths are 2.07 Å. Both Ta(1)-O(4) bond lengths are 2.06 Å. There are six inequivalent O sites. In the first O site, O(1) is bonded to two equivalent Er(2) and two equivalent Er(3) atoms to form OEr4 tetrahedra that share corners with two equivalent O(1)Er4 tetrahedra, corners with four equivalent O(3)Er3Ta tetrahedra, corners with four equivalent O(2)Er4Mg trigonal bipyramids,  an edgeedge with one O(1)Er4 tetrahedra, and edges with two equivalent O(3)Er3Ta tetrahedra. In the second O site, O(2) is bonded to one Mg(1), two equivalent Er(1), and two equivalent Er(2) atoms to form distorted OEr4Mg trigonal bipyramids that share corners with four equivalent O(3)Er3Ta tetrahedra, corners with four equivalent O(1)Er4 tetrahedra, corners with two equivalent O(2)Er4Mg trigonal bipyramids, edges with two equivalent O(3)Er3Ta tetrahedra, and  a faceface with one O(2)Er4Mg trigonal bipyramid. In the third O site, O(3) is bonded to one Er(1), one Er(2), one Er(3), and one Ta(1) atom to form distorted OEr3Ta tetrahedra that share corners with two equivalent O(3)Er3Ta tetrahedra, corners with two equivalent O(1)Er4 tetrahedra, corners with two equivalent O(2)Er4Mg trigonal bipyramids,  an edgeedge with one O(3)Er3Ta tetrahedra,  an edgeedge with one O(1)Er4 tetrahedra, and  an edgeedge with one O(2)Er4Mg trigonal bipyramid. In the fourth O site, O(4) is bonded in a 4-coordinate geometry to one Mg(1), one Er(1), one Er(3), and one Ta(1) atom. In the fifth O site, O(5) is bonded in a 4-coordinate geometry to two equivalent Er(1) and two equivalent Ta(1) atoms. In the sixth O site, O(6) is bonded in a 4-coordinate geometry to two equivalent Er(3) and two equivalent Ta(1) atoms.