MgEr2Nb2O8 crystallizes in the monoclinic P2 space group. Mg(1) is bonded to two equivalent O(1), two equivalent O(2), and two equivalent O(4) atoms to form MgO6 octahedra that share corners with four equivalent Nb(2)O6 octahedra, edges with two equivalent Er(1)O6 octahedra, edges with two equivalent Er(2)O6 octahedra, and faces with two equivalent Nb(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 37-49°. There are two inequivalent Er sites. In the first Er site, Er(1) is bonded to two equivalent O(1), two equivalent O(2), and two equivalent O(3) atoms to form ErO6 octahedra that share corners with four equivalent Nb(1)O6 octahedra, corners with four equivalent Nb(2)O6 octahedra, edges with two equivalent Mg(1)O6 octahedra, and edges with two equivalent Er(2)O6 octahedra. The corner-sharing octahedral tilt angles range from 42-53°. In the second Er site, Er(2) is bonded to two equivalent O(1), two equivalent O(3), and two equivalent O(4) atoms to form ErO6 octahedra that share corners with four equivalent Nb(1)O6 octahedra, corners with four equivalent Nb(2)O6 octahedra, edges with two equivalent Mg(1)O6 octahedra, and edges with two equivalent Er(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 43-54°. There are two inequivalent Nb sites. In the first Nb site, Nb(1) is bonded to two equivalent O(1), two equivalent O(2), and two equivalent O(4) atoms to form NbO6 octahedra that share corners with four equivalent Er(1)O6 octahedra, corners with four equivalent Er(2)O6 octahedra, edges with two equivalent Nb(2)O6 octahedra, and faces with two equivalent Mg(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 42-50°. In the second Nb site, Nb(2) is bonded to two equivalent O(2), two equivalent O(3), and two equivalent O(4) atoms to form NbO6 octahedra that share corners with four equivalent Mg(1)O6 octahedra, corners with four equivalent Er(1)O6 octahedra, corners with four equivalent Er(2)O6 octahedra, and edges with two equivalent Nb(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 37-54°. There are four inequivalent O sites. In the first O site, O(1) is bonded in a rectangular see-saw-like geometry to one Mg(1), one Er(1), one Er(2), and one Nb(1) atom. In the second O site, O(2) is bonded in a rectangular see-saw-like geometry to one Mg(1), one Er(1), one Nb(1), and one Nb(2) atom. In the third O site, O(3) is bonded in a distorted T-shaped geometry to one Er(1), one Er(2), and one Nb(2) atom. In the fourth O site, O(4) is bonded in a rectangular see-saw-like geometry to one Mg(1), one Er(2), one Nb(1), and one Nb(2) atom.
MgEr2Nb2O8 crystallizes in the monoclinic P2 space group. Mg(1) is bonded to two equivalent O(1), two equivalent O(2), and two equivalent O(4) atoms to form MgO6 octahedra that share corners with four equivalent Nb(2)O6 octahedra, edges with two equivalent Er(1)O6 octahedra, edges with two equivalent Er(2)O6 octahedra, and faces with two equivalent Nb(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 37-49°. Both Mg(1)-O(1) bond lengths are 2.04 Å. Both Mg(1)-O(2) bond lengths are 2.11 Å. Both Mg(1)-O(4) bond lengths are 2.12 Å. There are two inequivalent Er sites. In the first Er site, Er(1) is bonded to two equivalent O(1), two equivalent O(2), and two equivalent O(3) atoms to form ErO6 octahedra that share corners with four equivalent Nb(1)O6 octahedra, corners with four equivalent Nb(2)O6 octahedra, edges with two equivalent Mg(1)O6 octahedra, and edges with two equivalent Er(2)O6 octahedra. The corner-sharing octahedral tilt angles range from 42-53°. Both Er(1)-O(1) bond lengths are 2.27 Å. Both Er(1)-O(2) bond lengths are 2.26 Å. Both Er(1)-O(3) bond lengths are 2.25 Å. In the second Er site, Er(2) is bonded to two equivalent O(1), two equivalent O(3), and two equivalent O(4) atoms to form ErO6 octahedra that share corners with four equivalent Nb(1)O6 octahedra, corners with four equivalent Nb(2)O6 octahedra, edges with two equivalent Mg(1)O6 octahedra, and edges with two equivalent Er(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 43-54°. Both Er(2)-O(1) bond lengths are 2.21 Å. Both Er(2)-O(3) bond lengths are 2.35 Å. Both Er(2)-O(4) bond lengths are 2.25 Å. There are two inequivalent Nb sites. In the first Nb site, Nb(1) is bonded to two equivalent O(1), two equivalent O(2), and two equivalent O(4) atoms to form NbO6 octahedra that share corners with four equivalent Er(1)O6 octahedra, corners with four equivalent Er(2)O6 octahedra, edges with two equivalent Nb(2)O6 octahedra, and faces with two equivalent Mg(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 42-50°. Both Nb(1)-O(1) bond lengths are 2.09 Å. Both Nb(1)-O(2) bond lengths are 2.09 Å. Both Nb(1)-O(4) bond lengths are 2.12 Å. In the second Nb site, Nb(2) is bonded to two equivalent O(2), two equivalent O(3), and two equivalent O(4) atoms to form NbO6 octahedra that share corners with four equivalent Mg(1)O6 octahedra, corners with four equivalent Er(1)O6 octahedra, corners with four equivalent Er(2)O6 octahedra, and edges with two equivalent Nb(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 37-54°. Both Nb(2)-O(2) bond lengths are 2.26 Å. Both Nb(2)-O(3) bond lengths are 1.98 Å. Both Nb(2)-O(4) bond lengths are 2.25 Å. There are four inequivalent O sites. In the first O site, O(1) is bonded in a rectangular see-saw-like geometry to one Mg(1), one Er(1), one Er(2), and one Nb(1) atom. In the second O site, O(2) is bonded in a rectangular see-saw-like geometry to one Mg(1), one Er(1), one Nb(1), and one Nb(2) atom. In the third O site, O(3) is bonded in a distorted T-shaped geometry to one Er(1), one Er(2), and one Nb(2) atom. In the fourth O site, O(4) is bonded in a rectangular see-saw-like geometry to one Mg(1), one Er(2), one Nb(1), and one Nb(2) atom.