U2Sm2Se3O4 crystallizes in the orthorhombic Pbam space group. U(1) is bonded in a 4-coordinate geometry to two equivalent Se(1), two equivalent Se(2), two equivalent O(1), and two equivalent O(2) atoms. Sm(1) is bonded in a 8-coordinate geometry to one Se(2), four equivalent Se(1), one O(1), and two equivalent O(2) atoms. There are two inequivalent Se sites. In the first Se site, Se(1) is bonded in a 6-coordinate geometry to two equivalent U(1) and four equivalent Sm(1) atoms. In the second Se site, Se(2) is bonded to four equivalent U(1) and two equivalent Sm(1) atoms to form distorted SeSm2U4 octahedra that share corners with four equivalent O(2)Sm2U2 tetrahedra, edges with two equivalent Se(2)Sm2U4 octahedra, and edges with four equivalent O(2)Sm2U2 tetrahedra. There are two inequivalent O sites. In the first O site, O(1) is bonded in a distorted trigonal planar geometry to two equivalent U(1) and one Sm(1) atom. In the second O site, O(2) is bonded to two equivalent U(1) and two equivalent Sm(1) atoms to form distorted OSm2U2 tetrahedra that share corners with two equivalent Se(2)Sm2U4 octahedra, corners with two equivalent O(2)Sm2U2 tetrahedra, edges with two equivalent Se(2)Sm2U4 octahedra, and  an edgeedge with one O(2)Sm2U2 tetrahedra. The corner-sharing octahedral tilt angles are 50°.
U2Sm2Se3O4 crystallizes in the orthorhombic Pbam space group. U(1) is bonded in a 4-coordinate geometry to two equivalent Se(1), two equivalent Se(2), two equivalent O(1), and two equivalent O(2) atoms. There is one shorter (3.02 Å) and one longer (3.33 Å) U(1)-Se(1) bond length. Both U(1)-Se(2) bond lengths are 3.11 Å. Both U(1)-O(1) bond lengths are 2.16 Å. There is one shorter (2.20 Å) and one longer (2.43 Å) U(1)-O(2) bond length. Sm(1) is bonded in a 8-coordinate geometry to one Se(2), four equivalent Se(1), one O(1), and two equivalent O(2) atoms. The Sm(1)-Se(2) bond length is 3.06 Å. There are two shorter (3.06 Å) and two longer (3.10 Å) Sm(1)-Se(1) bond lengths. The Sm(1)-O(1) bond length is 2.35 Å. Both Sm(1)-O(2) bond lengths are 2.42 Å. There are two inequivalent Se sites. In the first Se site, Se(1) is bonded in a 6-coordinate geometry to two equivalent U(1) and four equivalent Sm(1) atoms. In the second Se site, Se(2) is bonded to four equivalent U(1) and two equivalent Sm(1) atoms to form distorted SeSm2U4 octahedra that share corners with four equivalent O(2)Sm2U2 tetrahedra, edges with two equivalent Se(2)Sm2U4 octahedra, and edges with four equivalent O(2)Sm2U2 tetrahedra. There are two inequivalent O sites. In the first O site, O(1) is bonded in a distorted trigonal planar geometry to two equivalent U(1) and one Sm(1) atom. In the second O site, O(2) is bonded to two equivalent U(1) and two equivalent Sm(1) atoms to form distorted OSm2U2 tetrahedra that share corners with two equivalent Se(2)Sm2U4 octahedra, corners with two equivalent O(2)Sm2U2 tetrahedra, edges with two equivalent Se(2)Sm2U4 octahedra, and  an edgeedge with one O(2)Sm2U2 tetrahedra. The corner-sharing octahedral tilt angles are 50°.