Er2O3 crystallizes in the trigonal P-3m1 space group. Er(1) is bonded in a 7-coordinate geometry to three equivalent O(2) and four equivalent O(1) atoms. There are two inequivalent O sites. In the first O site, O(1) is bonded to four equivalent Er(1) atoms to form OEr4 tetrahedra that share corners with six equivalent O(2)Er6 octahedra, corners with six equivalent O(1)Er4 tetrahedra, edges with three equivalent O(2)Er6 octahedra, and edges with three equivalent O(1)Er4 tetrahedra. The corner-sharing octahedral tilt angles range from 19-55°. In the second O site, O(2) is bonded to six equivalent Er(1) atoms to form OEr6 octahedra that share corners with twelve equivalent O(1)Er4 tetrahedra, edges with six equivalent O(2)Er6 octahedra, and edges with six equivalent O(1)Er4 tetrahedra.
Er2O3 crystallizes in the trigonal P-3m1 space group. Er(1) is bonded in a 7-coordinate geometry to three equivalent O(2) and four equivalent O(1) atoms. All Er(1)-O(2) bond lengths are 2.53 Å. There are three shorter (2.17 Å) and one longer (2.28 Å) Er(1)-O(1) bond length. There are two inequivalent O sites. In the first O site, O(1) is bonded to four equivalent Er(1) atoms to form OEr4 tetrahedra that share corners with six equivalent O(2)Er6 octahedra, corners with six equivalent O(1)Er4 tetrahedra, edges with three equivalent O(2)Er6 octahedra, and edges with three equivalent O(1)Er4 tetrahedra. The corner-sharing octahedral tilt angles range from 19-55°. In the second O site, O(2) is bonded to six equivalent Er(1) atoms to form OEr6 octahedra that share corners with twelve equivalent O(1)Er4 tetrahedra, edges with six equivalent O(2)Er6 octahedra, and edges with six equivalent O(1)Er4 tetrahedra.