K2Y4Cu4Se9 crystallizes in the monoclinic C2/m space group. K(1) is bonded in a 7-coordinate geometry to one Se(1), two equivalent Se(2), two equivalent Se(3), and two equivalent Se(4) atoms. There are two inequivalent Y sites. In the first Y site, Y(1) is bonded to one Se(3), two equivalent Se(2), and three equivalent Se(1) atoms to form YSe6 octahedra that share  a cornercorner with one Y(2)Se6 octahedra, corners with four equivalent Cu(1)Se4 tetrahedra, edges with four equivalent Y(1)Se6 octahedra, edges with two equivalent Cu(2)Se4 tetrahedra, and edges with three equivalent Cu(1)Se4 tetrahedra. The corner-sharing octahedral tilt angles are 39°. In the second Y site, Y(2) is bonded to one Se(3), two equivalent Se(5), and three equivalent Se(4) atoms to form YSe6 octahedra that share  a cornercorner with one Y(1)Se6 octahedra, corners with two equivalent Y(2)Se6 octahedra,  a cornercorner with one Cu(2)Se4 tetrahedra, edges with five equivalent Y(2)Se6 octahedra, and edges with two equivalent Cu(2)Se4 tetrahedra. The corner-sharing octahedral tilt angles range from 0-39°. There are two inequivalent Cu sites. In the first Cu site, Cu(1) is bonded to two equivalent Se(1) and two equivalent Se(2) atoms to form distorted CuSe4 tetrahedra that share corners with four equivalent Y(1)Se6 octahedra, corners with two equivalent Cu(1)Se4 tetrahedra, corners with two equivalent Cu(2)Se4 tetrahedra, edges with three equivalent Y(1)Se6 octahedra, and  an edgeedge with one Cu(1)Se4 tetrahedra. The corner-sharing octahedral tilt angles range from 19-63°. In the second Cu site, Cu(2) is bonded to one Se(2), one Se(4), and two equivalent Se(3) atoms to form CuSe4 tetrahedra that share  a cornercorner with one Y(2)Se6 octahedra, corners with two equivalent Cu(1)Se4 tetrahedra, corners with two equivalent Cu(2)Se4 tetrahedra, edges with two equivalent Y(1)Se6 octahedra, and edges with two equivalent Y(2)Se6 octahedra. The corner-sharing octahedral tilt angles are 8°. There are five inequivalent Se sites. In the first Se site, Se(1) is bonded to one K(1), three equivalent Y(1), and two equivalent Cu(1) atoms to form distorted SeKY3Cu2 octahedra that share corners with two equivalent Se(4)K2Y3Cu octahedra and edges with four equivalent Se(1)KY3Cu2 octahedra. The corner-sharing octahedral tilt angles are 41°. In the second Se site, Se(2) is bonded in a 7-coordinate geometry to two equivalent K(1), two equivalent Y(1), one Cu(2), and two equivalent Cu(1) atoms. In the third Se site, Se(3) is bonded in a 6-coordinate geometry to two equivalent K(1), one Y(1), one Y(2), and two equivalent Cu(2) atoms. In the fourth Se site, Se(4) is bonded to two equivalent K(1), three equivalent Y(2), and one Cu(2) atom to form distorted SeK2Y3Cu octahedra that share corners with two equivalent Se(1)KY3Cu2 octahedra and edges with four equivalent Se(4)K2Y3Cu octahedra. The corner-sharing octahedral tilt angles are 41°. In the fifth Se site, Se(5) is bonded in a square co-planar geometry to four equivalent Y(2) atoms.
K2Y4Cu4Se9 crystallizes in the monoclinic C2/m space group. K(1) is bonded in a 7-coordinate geometry to one Se(1), two equivalent Se(2), two equivalent Se(3), and two equivalent Se(4) atoms. The K(1)-Se(1) bond length is 3.27 Å. Both K(1)-Se(2) bond lengths are 3.51 Å. Both K(1)-Se(3) bond lengths are 3.31 Å. Both K(1)-Se(4) bond lengths are 3.31 Å. There are two inequivalent Y sites. In the first Y site, Y(1) is bonded to one Se(3), two equivalent Se(2), and three equivalent Se(1) atoms to form YSe6 octahedra that share  a cornercorner with one Y(2)Se6 octahedra, corners with four equivalent Cu(1)Se4 tetrahedra, edges with four equivalent Y(1)Se6 octahedra, edges with two equivalent Cu(2)Se4 tetrahedra, and edges with three equivalent Cu(1)Se4 tetrahedra. The corner-sharing octahedral tilt angles are 39°. The Y(1)-Se(3) bond length is 2.84 Å. Both Y(1)-Se(2) bond lengths are 2.86 Å. There is one shorter (2.86 Å) and two longer (2.94 Å) Y(1)-Se(1) bond lengths. In the second Y site, Y(2) is bonded to one Se(3), two equivalent Se(5), and three equivalent Se(4) atoms to form YSe6 octahedra that share  a cornercorner with one Y(1)Se6 octahedra, corners with two equivalent Y(2)Se6 octahedra,  a cornercorner with one Cu(2)Se4 tetrahedra, edges with five equivalent Y(2)Se6 octahedra, and edges with two equivalent Cu(2)Se4 tetrahedra. The corner-sharing octahedral tilt angles range from 0-39°. The Y(2)-Se(3) bond length is 2.82 Å. Both Y(2)-Se(5) bond lengths are 2.95 Å. There are two shorter (2.87 Å) and one longer (2.92 Å) Y(2)-Se(4) bond length. There are two inequivalent Cu sites. In the first Cu site, Cu(1) is bonded to two equivalent Se(1) and two equivalent Se(2) atoms to form distorted CuSe4 tetrahedra that share corners with four equivalent Y(1)Se6 octahedra, corners with two equivalent Cu(1)Se4 tetrahedra, corners with two equivalent Cu(2)Se4 tetrahedra, edges with three equivalent Y(1)Se6 octahedra, and  an edgeedge with one Cu(1)Se4 tetrahedra. The corner-sharing octahedral tilt angles range from 19-63°. Both Cu(1)-Se(1) bond lengths are 2.47 Å. There is one shorter (2.51 Å) and one longer (2.59 Å) Cu(1)-Se(2) bond length. In the second Cu site, Cu(2) is bonded to one Se(2), one Se(4), and two equivalent Se(3) atoms to form CuSe4 tetrahedra that share  a cornercorner with one Y(2)Se6 octahedra, corners with two equivalent Cu(1)Se4 tetrahedra, corners with two equivalent Cu(2)Se4 tetrahedra, edges with two equivalent Y(1)Se6 octahedra, and edges with two equivalent Y(2)Se6 octahedra. The corner-sharing octahedral tilt angles are 8°. The Cu(2)-Se(2) bond length is 2.63 Å. The Cu(2)-Se(4) bond length is 2.49 Å. Both Cu(2)-Se(3) bond lengths are 2.46 Å. There are five inequivalent Se sites. In the first Se site, Se(1) is bonded to one K(1), three equivalent Y(1), and two equivalent Cu(1) atoms to form distorted SeKY3Cu2 octahedra that share corners with two equivalent Se(4)K2Y3Cu octahedra and edges with four equivalent Se(1)KY3Cu2 octahedra. The corner-sharing octahedral tilt angles are 41°. In the second Se site, Se(2) is bonded in a 7-coordinate geometry to two equivalent K(1), two equivalent Y(1), one Cu(2), and two equivalent Cu(1) atoms. In the third Se site, Se(3) is bonded in a 6-coordinate geometry to two equivalent K(1), one Y(1), one Y(2), and two equivalent Cu(2) atoms. In the fourth Se site, Se(4) is bonded to two equivalent K(1), three equivalent Y(2), and one Cu(2) atom to form distorted SeK2Y3Cu octahedra that share corners with two equivalent Se(1)KY3Cu2 octahedra and edges with four equivalent Se(4)K2Y3Cu octahedra. The corner-sharing octahedral tilt angles are 41°. In the fifth Se site, Se(5) is bonded in a square co-planar geometry to four equivalent Y(2) atoms.