U8NiSe17 crystallizes in the monoclinic C2/m space group. There are three inequivalent U sites. In the first U site, U(1) is bonded in a 8-coordinate geometry to one Se(2), one Se(3), one Se(4), one Se(5), one Se(6), one Se(7), and two equivalent Se(1) atoms. In the second U site, U(2) is bonded in a 8-coordinate geometry to one Se(3), one Se(4), one Se(5), one Se(7), and four equivalent Se(1) atoms. In the third U site, U(3) is bonded in a 8-coordinate geometry to one Se(3), one Se(5), two equivalent Se(6), and four equivalent Se(2) atoms. Ni(1) is bonded in an octahedral geometry to two equivalent Se(4) and four equivalent Se(2) atoms. There are seven inequivalent Se sites. In the first Se site, Se(6) is bonded to two equivalent U(1) and two equivalent U(3) atoms to form SeU4 trigonal pyramids that share corners with two equivalent Se(7)U6 octahedra, corners with six equivalent Se(2)U3Ni tetrahedra, edges with two equivalent Se(2)U3Ni tetrahedra, and  an edgeedge with one Se(6)U4 trigonal pyramid. The corner-sharing octahedral tilt angles are 38°. In the second Se site, Se(7) is bonded to two equivalent U(2) and four equivalent U(1) atoms to form SeU6 octahedra that share corners with four equivalent Se(2)U3Ni tetrahedra and corners with four equivalent Se(6)U4 trigonal pyramids. In the third Se site, Se(1) is bonded in a rectangular see-saw-like geometry to two equivalent U(1) and two equivalent U(2) atoms. In the fourth Se site, Se(2) is bonded to one U(1), two equivalent U(3), and one Ni(1) atom to form SeU3Ni tetrahedra that share  a cornercorner with one Se(7)U6 octahedra, corners with five equivalent Se(2)U3Ni tetrahedra, corners with three equivalent Se(6)U4 trigonal pyramids, edges with two equivalent Se(2)U3Ni tetrahedra, and  an edgeedge with one Se(6)U4 trigonal pyramid. The corner-sharing octahedral tilt angles are 53°. In the fifth Se site, Se(3) is bonded in a rectangular see-saw-like geometry to one U(2), one U(3), and two equivalent U(1) atoms. In the sixth Se site, Se(4) is bonded in a distorted rectangular see-saw-like geometry to one U(2), two equivalent U(1), and one Ni(1) atom. In the seventh Se site, Se(5) is bonded in a rectangular see-saw-like geometry to one U(2), one U(3), and two equivalent U(1) atoms.
U8NiSe17 crystallizes in the monoclinic C2/m space group. There are three inequivalent U sites. In the first U site, U(1) is bonded in a 8-coordinate geometry to one Se(2), one Se(3), one Se(4), one Se(5), one Se(6), one Se(7), and two equivalent Se(1) atoms. The U(1)-Se(2) bond length is 2.95 Å. The U(1)-Se(3) bond length is 2.99 Å. The U(1)-Se(4) bond length is 2.87 Å. The U(1)-Se(5) bond length is 2.96 Å. The U(1)-Se(6) bond length is 2.98 Å. The U(1)-Se(7) bond length is 3.08 Å. There is one shorter (2.94 Å) and one longer (3.02 Å) U(1)-Se(1) bond length. In the second U site, U(2) is bonded in a 8-coordinate geometry to one Se(3), one Se(4), one Se(5), one Se(7), and four equivalent Se(1) atoms. The U(2)-Se(3) bond length is 2.89 Å. The U(2)-Se(4) bond length is 2.94 Å. The U(2)-Se(5) bond length is 2.91 Å. The U(2)-Se(7) bond length is 2.98 Å. There are two shorter (2.89 Å) and two longer (2.92 Å) U(2)-Se(1) bond lengths. In the third U site, U(3) is bonded in a 8-coordinate geometry to one Se(3), one Se(5), two equivalent Se(6), and four equivalent Se(2) atoms. The U(3)-Se(3) bond length is 2.97 Å. The U(3)-Se(5) bond length is 2.93 Å. There is one shorter (3.08 Å) and one longer (3.14 Å) U(3)-Se(6) bond length. There are two shorter (2.83 Å) and two longer (3.08 Å) U(3)-Se(2) bond lengths. Ni(1) is bonded in an octahedral geometry to two equivalent Se(4) and four equivalent Se(2) atoms. Both Ni(1)-Se(4) bond lengths are 2.41 Å. All Ni(1)-Se(2) bond lengths are 2.57 Å. There are seven inequivalent Se sites. In the first Se site, Se(6) is bonded to two equivalent U(1) and two equivalent U(3) atoms to form SeU4 trigonal pyramids that share corners with two equivalent Se(7)U6 octahedra, corners with six equivalent Se(2)U3Ni tetrahedra, edges with two equivalent Se(2)U3Ni tetrahedra, and  an edgeedge with one Se(6)U4 trigonal pyramid. The corner-sharing octahedral tilt angles are 38°. In the second Se site, Se(7) is bonded to two equivalent U(2) and four equivalent U(1) atoms to form SeU6 octahedra that share corners with four equivalent Se(2)U3Ni tetrahedra and corners with four equivalent Se(6)U4 trigonal pyramids. In the third Se site, Se(1) is bonded in a rectangular see-saw-like geometry to two equivalent U(1) and two equivalent U(2) atoms. In the fourth Se site, Se(2) is bonded to one U(1), two equivalent U(3), and one Ni(1) atom to form SeU3Ni tetrahedra that share  a cornercorner with one Se(7)U6 octahedra, corners with five equivalent Se(2)U3Ni tetrahedra, corners with three equivalent Se(6)U4 trigonal pyramids, edges with two equivalent Se(2)U3Ni tetrahedra, and  an edgeedge with one Se(6)U4 trigonal pyramid. The corner-sharing octahedral tilt angles are 53°. In the fifth Se site, Se(3) is bonded in a rectangular see-saw-like geometry to one U(2), one U(3), and two equivalent U(1) atoms. In the sixth Se site, Se(4) is bonded in a distorted rectangular see-saw-like geometry to one U(2), two equivalent U(1), and one Ni(1) atom. In the seventh Se site, Se(5) is bonded in a rectangular see-saw-like geometry to one U(2), one U(3), and two equivalent U(1) atoms.