Er2O3 crystallizes in the monoclinic C2/m space group. There are three inequivalent Er sites. In the first Er site, Er(1) is bonded to one O(3), one O(5), two equivalent O(2), and two equivalent O(4) atoms to form a mixture of distorted corner and edge-sharing ErO6 octahedra. The corner-sharing octahedra are not tilted. In the second Er site, Er(2) is bonded in a 7-coordinate geometry to one O(1), one O(4), two equivalent O(5), and three equivalent O(3) atoms. In the third Er site, Er(3) is bonded in a 7-coordinate geometry to two equivalent O(2), two equivalent O(5), and three equivalent O(1) atoms. There are five inequivalent O sites. In the first O site, O(1) is bonded in a 4-coordinate geometry to one Er(2) and three equivalent Er(3) atoms. In the second O site, O(2) is bonded to two equivalent Er(1) and two equivalent Er(3) atoms to form OEr4 tetrahedra that share corners with two equivalent O(4)Er6 octahedra, corners with two equivalent O(5)Er5 square pyramids, corners with two equivalent O(2)Er4 tetrahedra, corners with two equivalent O(3)Er4 tetrahedra,  an edgeedge with one O(4)Er6 octahedra, edges with two equivalent O(5)Er5 square pyramids, and  an edgeedge with one O(2)Er4 tetrahedra. The corner-sharing octahedral tilt angles are 14°. In the third O site, O(3) is bonded to one Er(1) and three equivalent Er(2) atoms to form OEr4 tetrahedra that share  a cornercorner with one O(4)Er6 octahedra, corners with five equivalent O(5)Er5 square pyramids, corners with two equivalent O(2)Er4 tetrahedra, corners with two equivalent O(3)Er4 tetrahedra, edges with two equivalent O(4)Er6 octahedra,  an edgeedge with one O(5)Er5 square pyramid, and edges with two equivalent O(3)Er4 tetrahedra. The corner-sharing octahedral tilt angles are 49°. In the fourth O site, O(4) is bonded to two equivalent Er(2) and four equivalent Er(1) atoms to form OEr6 octahedra that share corners with two equivalent O(3)Er4 tetrahedra, corners with four equivalent O(2)Er4 tetrahedra, edges with two equivalent O(4)Er6 octahedra, edges with four equivalent O(5)Er5 square pyramids, edges with two equivalent O(2)Er4 tetrahedra, and edges with four equivalent O(3)Er4 tetrahedra. In the fifth O site, O(5) is bonded to one Er(1), two equivalent Er(2), and two equivalent Er(3) atoms to form distorted OEr5 square pyramids that share corners with two equivalent O(2)Er4 tetrahedra, corners with five equivalent O(3)Er4 tetrahedra, edges with two equivalent O(4)Er6 octahedra, edges with two equivalent O(5)Er5 square pyramids,  an edgeedge with one O(3)Er4 tetrahedra, and edges with two equivalent O(2)Er4 tetrahedra.
Er2O3 crystallizes in the monoclinic C2/m space group. There are three inequivalent Er sites. In the first Er site, Er(1) is bonded to one O(3), one O(5), two equivalent O(2), and two equivalent O(4) atoms to form a mixture of distorted corner and edge-sharing ErO6 octahedra. The corner-sharing octahedra are not tilted. The Er(1)-O(3) bond length is 2.16 Å. The Er(1)-O(5) bond length is 2.22 Å. Both Er(1)-O(2) bond lengths are 2.17 Å. Both Er(1)-O(4) bond lengths are 2.43 Å. In the second Er site, Er(2) is bonded in a 7-coordinate geometry to one O(1), one O(4), two equivalent O(5), and three equivalent O(3) atoms. The Er(2)-O(1) bond length is 2.26 Å. The Er(2)-O(4) bond length is 2.68 Å. Both Er(2)-O(5) bond lengths are 2.36 Å. There is one shorter (2.20 Å) and two longer (2.22 Å) Er(2)-O(3) bond lengths. In the third Er site, Er(3) is bonded in a 7-coordinate geometry to two equivalent O(2), two equivalent O(5), and three equivalent O(1) atoms. There is one shorter (2.22 Å) and one longer (2.37 Å) Er(3)-O(2) bond length. Both Er(3)-O(5) bond lengths are 2.43 Å. There are two shorter (2.20 Å) and one longer (2.53 Å) Er(3)-O(1) bond length. There are five inequivalent O sites. In the first O site, O(1) is bonded in a 4-coordinate geometry to one Er(2) and three equivalent Er(3) atoms. In the second O site, O(2) is bonded to two equivalent Er(1) and two equivalent Er(3) atoms to form OEr4 tetrahedra that share corners with two equivalent O(4)Er6 octahedra, corners with two equivalent O(5)Er5 square pyramids, corners with two equivalent O(2)Er4 tetrahedra, corners with two equivalent O(3)Er4 tetrahedra,  an edgeedge with one O(4)Er6 octahedra, edges with two equivalent O(5)Er5 square pyramids, and  an edgeedge with one O(2)Er4 tetrahedra. The corner-sharing octahedral tilt angles are 14°. In the third O site, O(3) is bonded to one Er(1) and three equivalent Er(2) atoms to form OEr4 tetrahedra that share  a cornercorner with one O(4)Er6 octahedra, corners with five equivalent O(5)Er5 square pyramids, corners with two equivalent O(2)Er4 tetrahedra, corners with two equivalent O(3)Er4 tetrahedra, edges with two equivalent O(4)Er6 octahedra,  an edgeedge with one O(5)Er5 square pyramid, and edges with two equivalent O(3)Er4 tetrahedra. The corner-sharing octahedral tilt angles are 49°. In the fourth O site, O(4) is bonded to two equivalent Er(2) and four equivalent Er(1) atoms to form OEr6 octahedra that share corners with two equivalent O(3)Er4 tetrahedra, corners with four equivalent O(2)Er4 tetrahedra, edges with two equivalent O(4)Er6 octahedra, edges with four equivalent O(5)Er5 square pyramids, edges with two equivalent O(2)Er4 tetrahedra, and edges with four equivalent O(3)Er4 tetrahedra. In the fifth O site, O(5) is bonded to one Er(1), two equivalent Er(2), and two equivalent Er(3) atoms to form distorted OEr5 square pyramids that share corners with two equivalent O(2)Er4 tetrahedra, corners with five equivalent O(3)Er4 tetrahedra, edges with two equivalent O(4)Er6 octahedra, edges with two equivalent O(5)Er5 square pyramids,  an edgeedge with one O(3)Er4 tetrahedra, and edges with two equivalent O(2)Er4 tetrahedra.