Sm2Se2O crystallizes in the monoclinic P2_1/c space group. There are two inequivalent Sm sites. In the first Sm site, Sm(1) is bonded in a 7-coordinate geometry to two equivalent Se(2), four equivalent Se(1), and one O(1) atom. In the second Sm site, Sm(2) is bonded in a 3-coordinate geometry to one Se(1), three equivalent Se(2), and three equivalent O(1) atoms. There are two inequivalent Se sites. In the first Se site, Se(1) is bonded to one Sm(2) and four equivalent Sm(1) atoms to form distorted SeSm5 trigonal bipyramids that share corners with three equivalent O(1)Sm4 tetrahedra, corners with four equivalent Se(1)Sm5 trigonal bipyramids, edges with two equivalent O(1)Sm4 tetrahedra, and edges with four equivalent Se(1)Sm5 trigonal bipyramids. In the second Se site, Se(2) is bonded in a 5-coordinate geometry to two equivalent Sm(1) and three equivalent Sm(2) atoms. O(1) is bonded to one Sm(1) and three equivalent Sm(2) atoms to form OSm4 tetrahedra that share corners with four equivalent O(1)Sm4 tetrahedra, corners with three equivalent Se(1)Sm5 trigonal bipyramids,  an edgeedge with one O(1)Sm4 tetrahedra, and edges with two equivalent Se(1)Sm5 trigonal bipyramids.
Sm2Se2O crystallizes in the monoclinic P2_1/c space group. There are two inequivalent Sm sites. In the first Sm site, Sm(1) is bonded in a 7-coordinate geometry to two equivalent Se(2), four equivalent Se(1), and one O(1) atom. There is one shorter (2.94 Å) and one longer (3.08 Å) Sm(1)-Se(2) bond length. There are a spread of Sm(1)-Se(1) bond distances ranging from 2.88-3.05 Å. The Sm(1)-O(1) bond length is 2.33 Å. In the second Sm site, Sm(2) is bonded in a 3-coordinate geometry to one Se(1), three equivalent Se(2), and three equivalent O(1) atoms. The Sm(2)-Se(1) bond length is 3.10 Å. There are a spread of Sm(2)-Se(2) bond distances ranging from 2.93-3.34 Å. There is one shorter (2.33 Å) and two longer (2.38 Å) Sm(2)-O(1) bond lengths. There are two inequivalent Se sites. In the first Se site, Se(1) is bonded to one Sm(2) and four equivalent Sm(1) atoms to form distorted SeSm5 trigonal bipyramids that share corners with three equivalent O(1)Sm4 tetrahedra, corners with four equivalent Se(1)Sm5 trigonal bipyramids, edges with two equivalent O(1)Sm4 tetrahedra, and edges with four equivalent Se(1)Sm5 trigonal bipyramids. In the second Se site, Se(2) is bonded in a 5-coordinate geometry to two equivalent Sm(1) and three equivalent Sm(2) atoms. O(1) is bonded to one Sm(1) and three equivalent Sm(2) atoms to form OSm4 tetrahedra that share corners with four equivalent O(1)Sm4 tetrahedra, corners with three equivalent Se(1)Sm5 trigonal bipyramids,  an edgeedge with one O(1)Sm4 tetrahedra, and edges with two equivalent Se(1)Sm5 trigonal bipyramids.