CaU2Se5 crystallizes in the orthorhombic Pnma space group. Ca(1) is bonded to one Se(2), one Se(3), one Se(4), and four equivalent Se(1) atoms to form distorted edge-sharing CaSe7 pentagonal bipyramids. 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 are four inequivalent Se sites. In the first Se site, Se(1) is bonded to two equivalent Ca(1) and two equivalent U(1) atoms to form distorted SeCa2U2 tetrahedra that share corners with two equivalent Se(4)CaU4 square pyramids, corners with four equivalent Se(3)CaU4 square pyramids, corners with six equivalent Se(1)Ca2U2 tetrahedra, corners with two equivalent Se(2)CaU4 trigonal bipyramids,  an edgeedge with one Se(3)CaU4 square pyramid, edges with two equivalent Se(4)CaU4 square pyramids,  an edgeedge with one Se(1)Ca2U2 tetrahedra, and edges with two equivalent Se(2)CaU4 trigonal bipyramids. In the second Se site, Se(2) is bonded to one Ca(1) and four equivalent U(1) atoms to form distorted SeCaU4 trigonal bipyramids that share  a cornercorner with one Se(4)CaU4 square pyramid, corners with five equivalent Se(3)CaU4 square pyramids, corners with four equivalent Se(1)Ca2U2 tetrahedra, corners with four equivalent Se(2)CaU4 trigonal bipyramids, edges with two equivalent Se(3)CaU4 square pyramids, edges with four equivalent Se(4)CaU4 square pyramids, and edges with four equivalent Se(1)Ca2U2 tetrahedra. In the third Se site, Se(3) is bonded to one Ca(1) and four equivalent U(1) atoms to form SeCaU4 square pyramids that share corners with four equivalent Se(4)CaU4 square pyramids, corners with eight equivalent Se(1)Ca2U2 tetrahedra, corners with five equivalent Se(2)CaU4 trigonal bipyramids,  an edgeedge with one Se(4)CaU4 square pyramid, edges with two equivalent Se(3)CaU4 square pyramids, edges with two equivalent Se(1)Ca2U2 tetrahedra, edges with two equivalent Se(2)CaU4 trigonal bipyramids, and  a faceface with one Se(4)CaU4 square pyramid. In the fourth Se site, Se(4) is bonded to one Ca(1) and four equivalent U(1) atoms to form SeCaU4 square pyramids that share corners with four equivalent Se(3)CaU4 square pyramids, corners with four equivalent Se(4)CaU4 square pyramids, corners with four equivalent Se(1)Ca2U2 tetrahedra,  a cornercorner with one Se(2)CaU4 trigonal bipyramid,  an edgeedge with one Se(3)CaU4 square pyramid, edges with four equivalent Se(1)Ca2U2 tetrahedra, edges with four equivalent Se(2)CaU4 trigonal bipyramids, and  a faceface with one Se(3)CaU4 square pyramid.
CaU2Se5 crystallizes in the orthorhombic Pnma space group. Ca(1) is bonded to one Se(2), one Se(3), one Se(4), and four equivalent Se(1) atoms to form distorted edge-sharing CaSe7 pentagonal bipyramids. The Ca(1)-Se(2) bond length is 2.97 Å. The Ca(1)-Se(3) bond length is 3.05 Å. The Ca(1)-Se(4) bond length is 2.93 Å. There are two shorter (2.81 Å) and two longer (2.90 Å) Ca(1)-Se(1) bond lengths. 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 (2.80 Å) and one longer (2.87 Å) U(1)-Se(1) bond length. There is one shorter (3.02 Å) and one longer (3.13 Å) U(1)-Se(2) bond length. There is one shorter (3.02 Å) and one longer (3.08 Å) U(1)-Se(3) bond length. There is one shorter (2.95 Å) and one longer (3.00 Å) U(1)-Se(4) bond length. There are four inequivalent Se sites. In the first Se site, Se(1) is bonded to two equivalent Ca(1) and two equivalent U(1) atoms to form distorted SeCa2U2 tetrahedra that share corners with two equivalent Se(4)CaU4 square pyramids, corners with four equivalent Se(3)CaU4 square pyramids, corners with six equivalent Se(1)Ca2U2 tetrahedra, corners with two equivalent Se(2)CaU4 trigonal bipyramids,  an edgeedge with one Se(3)CaU4 square pyramid, edges with two equivalent Se(4)CaU4 square pyramids,  an edgeedge with one Se(1)Ca2U2 tetrahedra, and edges with two equivalent Se(2)CaU4 trigonal bipyramids. In the second Se site, Se(2) is bonded to one Ca(1) and four equivalent U(1) atoms to form distorted SeCaU4 trigonal bipyramids that share  a cornercorner with one Se(4)CaU4 square pyramid, corners with five equivalent Se(3)CaU4 square pyramids, corners with four equivalent Se(1)Ca2U2 tetrahedra, corners with four equivalent Se(2)CaU4 trigonal bipyramids, edges with two equivalent Se(3)CaU4 square pyramids, edges with four equivalent Se(4)CaU4 square pyramids, and edges with four equivalent Se(1)Ca2U2 tetrahedra. In the third Se site, Se(3) is bonded to one Ca(1) and four equivalent U(1) atoms to form SeCaU4 square pyramids that share corners with four equivalent Se(4)CaU4 square pyramids, corners with eight equivalent Se(1)Ca2U2 tetrahedra, corners with five equivalent Se(2)CaU4 trigonal bipyramids,  an edgeedge with one Se(4)CaU4 square pyramid, edges with two equivalent Se(3)CaU4 square pyramids, edges with two equivalent Se(1)Ca2U2 tetrahedra, edges with two equivalent Se(2)CaU4 trigonal bipyramids, and  a faceface with one Se(4)CaU4 square pyramid. In the fourth Se site, Se(4) is bonded to one Ca(1) and four equivalent U(1) atoms to form SeCaU4 square pyramids that share corners with four equivalent Se(3)CaU4 square pyramids, corners with four equivalent Se(4)CaU4 square pyramids, corners with four equivalent Se(1)Ca2U2 tetrahedra,  a cornercorner with one Se(2)CaU4 trigonal bipyramid,  an edgeedge with one Se(3)CaU4 square pyramid, edges with four equivalent Se(1)Ca2U2 tetrahedra, edges with four equivalent Se(2)CaU4 trigonal bipyramids, and  a faceface with one Se(3)CaU4 square pyramid.