Er2S2O crystallizes in the monoclinic P2_1/c space group. There are two inequivalent Er sites. In the first Er site, Er(1) is bonded in a 7-coordinate geometry to two equivalent S(2), four equivalent S(1), and one O(1) atom. In the second Er site, Er(2) is bonded in a 3-coordinate geometry to one S(1), three equivalent S(2), and three equivalent O(1) atoms. There are two inequivalent S sites. In the first S site, S(1) is bonded to one Er(2) and four equivalent Er(1) atoms to form distorted SEr5 trigonal bipyramids that share corners with three equivalent O(1)Er4 tetrahedra, corners with four equivalent S(1)Er5 trigonal bipyramids, edges with two equivalent O(1)Er4 tetrahedra, and edges with four equivalent S(1)Er5 trigonal bipyramids. In the second S site, S(2) is bonded in a 5-coordinate geometry to two equivalent Er(1) and three equivalent Er(2) atoms. O(1) is bonded to one Er(1) and three equivalent Er(2) atoms to form OEr4 tetrahedra that share corners with four equivalent O(1)Er4 tetrahedra, corners with three equivalent S(1)Er5 trigonal bipyramids,  an edgeedge with one O(1)Er4 tetrahedra, and edges with two equivalent S(1)Er5 trigonal bipyramids.
Er2S2O crystallizes in the monoclinic P2_1/c space group. There are two inequivalent Er sites. In the first Er site, Er(1) is bonded in a 7-coordinate geometry to two equivalent S(2), four equivalent S(1), and one O(1) atom. There is one shorter (2.74 Å) and one longer (2.92 Å) Er(1)-S(2) bond length. There are a spread of Er(1)-S(1) bond distances ranging from 2.70-2.82 Å. The Er(1)-O(1) bond length is 2.22 Å. In the second Er site, Er(2) is bonded in a 3-coordinate geometry to one S(1), three equivalent S(2), and three equivalent O(1) atoms. The Er(2)-S(1) bond length is 2.91 Å. There are two shorter (2.75 Å) and one longer (3.16 Å) Er(2)-S(2) bond length. There are two shorter (2.22 Å) and one longer (2.25 Å) Er(2)-O(1) bond length. There are two inequivalent S sites. In the first S site, S(1) is bonded to one Er(2) and four equivalent Er(1) atoms to form distorted SEr5 trigonal bipyramids that share corners with three equivalent O(1)Er4 tetrahedra, corners with four equivalent S(1)Er5 trigonal bipyramids, edges with two equivalent O(1)Er4 tetrahedra, and edges with four equivalent S(1)Er5 trigonal bipyramids. In the second S site, S(2) is bonded in a 5-coordinate geometry to two equivalent Er(1) and three equivalent Er(2) atoms. O(1) is bonded to one Er(1) and three equivalent Er(2) atoms to form OEr4 tetrahedra that share corners with four equivalent O(1)Er4 tetrahedra, corners with three equivalent S(1)Er5 trigonal bipyramids,  an edgeedge with one O(1)Er4 tetrahedra, and edges with two equivalent S(1)Er5 trigonal bipyramids.