U2SmSe5 crystallizes in the orthorhombic Pnma space group. U(1) is bonded in a 8-coordinate geometry to two equivalent Se(1), two equivalent Se(2), two equivalent Se(3), and two equivalent Se(4) atoms. Sm(1) is bonded in a 7-coordinate geometry to one Se(1), one Se(2), one Se(3), and four equivalent Se(4) atoms. There are four inequivalent Se sites. In the first Se site, Se(1) is bonded to four equivalent U(1) and one Sm(1) atom to form distorted SeSmU4 square pyramids that share corners with four equivalent Se(1)SmU4 square pyramids, corners with four equivalent Se(3)SmU4 square pyramids,  a cornercorner with one Se(2)SmU4 trigonal bipyramid, corners with four equivalent Se(4)Sm2U2 trigonal pyramids,  an edgeedge with one Se(3)SmU4 square pyramid, edges with four equivalent Se(2)SmU4 trigonal bipyramids, edges with four equivalent Se(4)Sm2U2 trigonal pyramids, and  a faceface with one Se(3)SmU4 square pyramid. In the second Se site, Se(2) is bonded to four equivalent U(1) and one Sm(1) atom to form distorted SeSmU4 trigonal bipyramids that share  a cornercorner with one Se(1)SmU4 square pyramid, corners with five equivalent Se(3)SmU4 square pyramids, corners with four equivalent Se(2)SmU4 trigonal bipyramids, corners with four equivalent Se(4)Sm2U2 trigonal pyramids, edges with two equivalent Se(3)SmU4 square pyramids, edges with four equivalent Se(1)SmU4 square pyramids, and edges with four equivalent Se(4)Sm2U2 trigonal pyramids. In the third Se site, Se(3) is bonded to four equivalent U(1) and one Sm(1) atom to form SeSmU4 square pyramids that share corners with four equivalent Se(1)SmU4 square pyramids, corners with five equivalent Se(2)SmU4 trigonal bipyramids, corners with eight equivalent Se(4)Sm2U2 trigonal pyramids,  an edgeedge with one Se(1)SmU4 square pyramid, edges with two equivalent Se(3)SmU4 square pyramids, edges with two equivalent Se(2)SmU4 trigonal bipyramids, edges with two equivalent Se(4)Sm2U2 trigonal pyramids, and  a faceface with one Se(1)SmU4 square pyramid. In the fourth Se site, Se(4) is bonded to two equivalent U(1) and two equivalent Sm(1) atoms to form distorted SeSm2U2 trigonal pyramids that share corners with two equivalent Se(1)SmU4 square pyramids, corners with four equivalent Se(3)SmU4 square pyramids, corners with two equivalent Se(2)SmU4 trigonal bipyramids, corners with six equivalent Se(4)Sm2U2 trigonal pyramids,  an edgeedge with one Se(3)SmU4 square pyramid, edges with two equivalent Se(1)SmU4 square pyramids, edges with two equivalent Se(2)SmU4 trigonal bipyramids, and  an edgeedge with one Se(4)Sm2U2 trigonal pyramid.
U2SmSe5 crystallizes in the orthorhombic Pnma space group. U(1) is bonded in a 8-coordinate geometry to two equivalent Se(1), two equivalent Se(2), two equivalent Se(3), and two equivalent Se(4) atoms. There is one shorter (2.96 Å) and one longer (3.05 Å) U(1)-Se(1) bond length. There is one shorter (3.04 Å) and one longer (3.18 Å) U(1)-Se(2) bond length. There is one shorter (3.06 Å) and one longer (3.08 Å) U(1)-Se(3) bond length. There is one shorter (2.90 Å) and one longer (2.91 Å) U(1)-Se(4) bond length. Sm(1) is bonded in a 7-coordinate geometry to one Se(1), one Se(2), one Se(3), and four equivalent Se(4) atoms. The Sm(1)-Se(1) bond length is 2.93 Å. The Sm(1)-Se(2) bond length is 3.00 Å. The Sm(1)-Se(3) bond length is 3.04 Å. There are two shorter (2.82 Å) and two longer (2.92 Å) Sm(1)-Se(4) bond lengths. There are four inequivalent Se sites. In the first Se site, Se(1) is bonded to four equivalent U(1) and one Sm(1) atom to form distorted SeSmU4 square pyramids that share corners with four equivalent Se(1)SmU4 square pyramids, corners with four equivalent Se(3)SmU4 square pyramids,  a cornercorner with one Se(2)SmU4 trigonal bipyramid, corners with four equivalent Se(4)Sm2U2 trigonal pyramids,  an edgeedge with one Se(3)SmU4 square pyramid, edges with four equivalent Se(2)SmU4 trigonal bipyramids, edges with four equivalent Se(4)Sm2U2 trigonal pyramids, and  a faceface with one Se(3)SmU4 square pyramid. In the second Se site, Se(2) is bonded to four equivalent U(1) and one Sm(1) atom to form distorted SeSmU4 trigonal bipyramids that share  a cornercorner with one Se(1)SmU4 square pyramid, corners with five equivalent Se(3)SmU4 square pyramids, corners with four equivalent Se(2)SmU4 trigonal bipyramids, corners with four equivalent Se(4)Sm2U2 trigonal pyramids, edges with two equivalent Se(3)SmU4 square pyramids, edges with four equivalent Se(1)SmU4 square pyramids, and edges with four equivalent Se(4)Sm2U2 trigonal pyramids. In the third Se site, Se(3) is bonded to four equivalent U(1) and one Sm(1) atom to form SeSmU4 square pyramids that share corners with four equivalent Se(1)SmU4 square pyramids, corners with five equivalent Se(2)SmU4 trigonal bipyramids, corners with eight equivalent Se(4)Sm2U2 trigonal pyramids,  an edgeedge with one Se(1)SmU4 square pyramid, edges with two equivalent Se(3)SmU4 square pyramids, edges with two equivalent Se(2)SmU4 trigonal bipyramids, edges with two equivalent Se(4)Sm2U2 trigonal pyramids, and  a faceface with one Se(1)SmU4 square pyramid. In the fourth Se site, Se(4) is bonded to two equivalent U(1) and two equivalent Sm(1) atoms to form distorted SeSm2U2 trigonal pyramids that share corners with two equivalent Se(1)SmU4 square pyramids, corners with four equivalent Se(3)SmU4 square pyramids, corners with two equivalent Se(2)SmU4 trigonal bipyramids, corners with six equivalent Se(4)Sm2U2 trigonal pyramids,  an edgeedge with one Se(3)SmU4 square pyramid, edges with two equivalent Se(1)SmU4 square pyramids, edges with two equivalent Se(2)SmU4 trigonal bipyramids, and  an edgeedge with one Se(4)Sm2U2 trigonal pyramid.