Cu27Se20 crystallizes in the trigonal R3m space group. There are twenty-five inequivalent Cu sites. In the first Cu site, Cu(1) is bonded to one Se(3) and three equivalent Se(2) atoms to form a mixture of corner and edge-sharing CuSe4 tetrahedra. In the second Cu site, Cu(2) is bonded to one Se(1) and three equivalent Se(2) atoms to form a mixture of corner and edge-sharing CuSe4 tetrahedra. In the third Cu site, Cu(3) is bonded to one Se(5) and three equivalent Se(4) atoms to form a mixture of corner and edge-sharing CuSe4 tetrahedra. In the fourth Cu site, Cu(4) is bonded to one Se(2) and three equivalent Se(3) atoms to form a mixture of corner and edge-sharing CuSe4 tetrahedra. In the fifth Cu site, Cu(5) is bonded to one Se(3) and three equivalent Se(4) atoms to form a mixture of corner and edge-sharing CuSe4 tetrahedra. In the sixth Cu site, Cu(6) is bonded to one Se(4) and three equivalent Se(5) atoms to form a mixture of corner and edge-sharing CuSe4 tetrahedra. In the seventh Cu site, Cu(7) is bonded to one Se(5) and three equivalent Se(6) atoms to form corner-sharing CuSe4 tetrahedra. In the eighth Cu site, Cu(8) is bonded to one Se(6) and three equivalent Se(7) atoms to form corner-sharing CuSe4 tetrahedra. In the ninth Cu site, Cu(9) is bonded to one Se(7) and three equivalent Se(8,9,10) atoms to form corner-sharing CuSe4 tetrahedra. In the tenth Cu site, Cu(10,11,12) is bonded to four equivalent Se(8,9,10) atoms to form corner-sharing CuSe4 tetrahedra. In the eleventh Cu site, Cu(13) is bonded to one Se(11) and three equivalent Se(12) atoms to form corner-sharing CuSe4 tetrahedra. In the twelfth Cu site, Cu(14) is bonded to one Se(15) and three equivalent Se(14) atoms to form a mixture of corner and edge-sharing CuSe4 tetrahedra. In the thirteenth Cu site, Cu(15) is bonded to one Se(12) and three equivalent Se(13) atoms to form corner-sharing CuSe4 tetrahedra. In the fourteenth Cu site, Cu(16) is bonded to one Se(13) and three equivalent Se(14) atoms to form a mixture of corner and edge-sharing CuSe4 tetrahedra. In the fifteenth Cu site, Cu(17) is bonded to one Se(14) and three equivalent Se(15) atoms to form a mixture of corner and edge-sharing CuSe4 tetrahedra. In the sixteenth Cu site, Cu(18) is bonded to one Se(17) and three equivalent Se(16) atoms to form a mixture of corner and edge-sharing CuSe4 tetrahedra. In the seventeenth Cu site, Cu(19) is bonded to one Se(15) and three equivalent Se(16) atoms to form a mixture of corner and edge-sharing CuSe4 tetrahedra. In the eighteenth Cu site, Cu(20) is bonded to one Se(16) and three equivalent Se(17) atoms to form a mixture of corner and edge-sharing CuSe4 tetrahedra. In the nineteenth Cu site, Cu(21) is bonded to one Se(19) and three equivalent Se(18) atoms to form a mixture of corner and edge-sharing CuSe4 tetrahedra. In the twentieth Cu site, Cu(22) is bonded to one Se(17) and three equivalent Se(18) atoms to form a mixture of corner and edge-sharing CuSe4 tetrahedra. In the twenty-first Cu site, Cu(23) is bonded to one Se(20) and three equivalent Se(19) atoms to form a mixture of corner and edge-sharing CuSe4 tetrahedra. In the twenty-second Cu site, Cu(24) is bonded to one Se(1) and three equivalent Se(20) atoms to form a mixture of corner and edge-sharing CuSe4 tetrahedra. In the twenty-third Cu site, Cu(25) is bonded to one Se(18) and three equivalent Se(19) atoms to form a mixture of corner and edge-sharing CuSe4 tetrahedra. In the twenty-fourth Cu site, Cu(26) is bonded to one Se(19) and three equivalent Se(20) atoms to form a mixture of corner and edge-sharing CuSe4 tetrahedra. In the twenty-fifth Cu site, Cu(27) is bonded to one Se(20) and three equivalent Se(1) atoms to form a mixture of corner and edge-sharing CuSe4 tetrahedra. There are eighteen inequivalent Se sites. In the first Se site, Se(1) is bonded in a distorted pentagonal planar geometry to one Cu(2), one Cu(24), and three equivalent Cu(27) atoms. In the second Se site, Se(2) is bonded in a 7-coordinate geometry to one Cu(4), three equivalent Cu(1), and three equivalent Cu(2) atoms. In the third Se site, Se(3) is bonded in a distorted pentagonal planar geometry to one Cu(1), one Cu(5), and three equivalent Cu(4) atoms. In the fourth Se site, Se(4) is bonded in a 7-coordinate geometry to one Cu(6), three equivalent Cu(3), and three equivalent Cu(5) atoms. In the fifth Se site, Se(5) is bonded in a distorted pentagonal planar geometry to one Cu(3), one Cu(7), and three equivalent Cu(6) atoms. In the sixth Se site, Se(6) is bonded to one Cu(8) and three equivalent Cu(7) atoms to form corner-sharing SeCu4 tetrahedra. In the seventh Se site, Se(7) is bonded to one Cu(9) and three equivalent Cu(8) atoms to form corner-sharing SeCu4 tetrahedra. In the eighth Se site, Se(8,9,10) is bonded to one Cu(10,11,12) and three equivalent Cu(9) atoms to form corner-sharing SeCu4 tetrahedra. In the ninth Se site, Se(11) is bonded to one Cu(13) and three equivalent Cu(10,11,12) atoms to form corner-sharing SeCu4 tetrahedra. In the tenth Se site, Se(12) is bonded to one Cu(15) and three equivalent Cu(13) atoms to form corner-sharing SeCu4 tetrahedra. In the eleventh Se site, Se(13) is bonded to one Cu(16) and three equivalent Cu(15) atoms to form corner-sharing SeCu4 tetrahedra. In the twelfth Se site, Se(14) is bonded in a 7-coordinate geometry to one Cu(17), three equivalent Cu(14), and three equivalent Cu(16) atoms. In the thirteenth Se site, Se(15) is bonded in a distorted pentagonal planar geometry to one Cu(14), one Cu(19), and three equivalent Cu(17) atoms. In the fourteenth Se site, Se(16) is bonded in a 7-coordinate geometry to one Cu(20), three equivalent Cu(18), and three equivalent Cu(19) atoms. In the fifteenth Se site, Se(17) is bonded in a distorted pentagonal planar geometry to one Cu(18), one Cu(22), and three equivalent Cu(20) atoms. In the sixteenth Se site, Se(18) is bonded to one Cu(25), three equivalent Cu(21), and three equivalent Cu(22) atoms to form distorted edge-sharing SeCu7 hexagonal pyramids. In the seventeenth Se site, Se(19) is bonded in a body-centered cubic geometry to one Cu(21), one Cu(26), three equivalent Cu(23), and three equivalent Cu(25) atoms. In the eighteenth Se site, Se(20) is bonded in a body-centered cubic geometry to one Cu(23), one Cu(27), three equivalent Cu(24), and three equivalent Cu(26) atoms.
Cu27Se20 crystallizes in the trigonal R3m space group. There are twenty-five inequivalent Cu sites. In the first Cu site, Cu(1) is bonded to one Se(3) and three equivalent Se(2) atoms to form a mixture of corner and edge-sharing CuSe4 tetrahedra. The Cu(1)-Se(3) bond length is 2.42 Å. All Cu(1)-Se(2) bond lengths are 2.50 Å. In the second Cu site, Cu(2) is bonded to one Se(1) and three equivalent Se(2) atoms to form a mixture of corner and edge-sharing CuSe4 tetrahedra. The Cu(2)-Se(1) bond length is 2.37 Å. All Cu(2)-Se(2) bond lengths are 2.45 Å. In the third Cu site, Cu(3) is bonded to one Se(5) and three equivalent Se(4) atoms to form a mixture of corner and edge-sharing CuSe4 tetrahedra. The Cu(3)-Se(5) bond length is 2.44 Å. All Cu(3)-Se(4) bond lengths are 2.50 Å. In the fourth Cu site, Cu(4) is bonded to one Se(2) and three equivalent Se(3) atoms to form a mixture of corner and edge-sharing CuSe4 tetrahedra. The Cu(4)-Se(2) bond length is 2.57 Å. All Cu(4)-Se(3) bond lengths are 2.45 Å. In the fifth Cu site, Cu(5) is bonded to one Se(3) and three equivalent Se(4) atoms to form a mixture of corner and edge-sharing CuSe4 tetrahedra. The Cu(5)-Se(3) bond length is 2.36 Å. All Cu(5)-Se(4) bond lengths are 2.45 Å. In the sixth Cu site, Cu(6) is bonded to one Se(4) and three equivalent Se(5) atoms to form a mixture of corner and edge-sharing CuSe4 tetrahedra. The Cu(6)-Se(4) bond length is 2.56 Å. All Cu(6)-Se(5) bond lengths are 2.46 Å. In the seventh Cu site, Cu(7) is bonded to one Se(5) and three equivalent Se(6) atoms to form corner-sharing CuSe4 tetrahedra. The Cu(7)-Se(5) bond length is 2.46 Å. All Cu(7)-Se(6) bond lengths are 2.43 Å. In the eighth Cu site, Cu(8) is bonded to one Se(6) and three equivalent Se(7) atoms to form corner-sharing CuSe4 tetrahedra. The Cu(8)-Se(6) bond length is 2.44 Å. All Cu(8)-Se(7) bond lengths are 2.46 Å. In the ninth Cu site, Cu(9) is bonded to one Se(7) and three equivalent Se(8,9,10) atoms to form corner-sharing CuSe4 tetrahedra. The Cu(9)-Se(7) bond length is 2.44 Å. All Cu(9)-Se(8,9,10) bond lengths are 2.46 Å. In the tenth Cu site, Cu(10,11,12) is bonded to four equivalent Se(8,9,10) atoms to form corner-sharing CuSe4 tetrahedra. There is one shorter (2.44 Å) and three longer (2.46 Å) Cu(10,11,12)-Se(8,9,10) bond lengths. In the eleventh Cu site, Cu(13) is bonded to one Se(11) and three equivalent Se(12) atoms to form corner-sharing CuSe4 tetrahedra. The Cu(13)-Se(11) bond length is 2.45 Å. All Cu(13)-Se(12) bond lengths are 2.45 Å. In the twelfth Cu site, Cu(14) is bonded to one Se(15) and three equivalent Se(14) atoms to form a mixture of corner and edge-sharing CuSe4 tetrahedra. The Cu(14)-Se(15) bond length is 2.42 Å. All Cu(14)-Se(14) bond lengths are 2.50 Å. In the thirteenth Cu site, Cu(15) is bonded to one Se(12) and three equivalent Se(13) atoms to form corner-sharing CuSe4 tetrahedra. The Cu(15)-Se(12) bond length is 2.45 Å. All Cu(15)-Se(13) bond lengths are 2.44 Å. In the fourteenth Cu site, Cu(16) is bonded to one Se(13) and three equivalent Se(14) atoms to form a mixture of corner and edge-sharing CuSe4 tetrahedra. The Cu(16)-Se(13) bond length is 2.36 Å. All Cu(16)-Se(14) bond lengths are 2.48 Å. In the fifteenth Cu site, Cu(17) is bonded to one Se(14) and three equivalent Se(15) atoms to form a mixture of corner and edge-sharing CuSe4 tetrahedra. The Cu(17)-Se(14) bond length is 2.56 Å. All Cu(17)-Se(15) bond lengths are 2.45 Å. In the sixteenth Cu site, Cu(18) is bonded to one Se(17) and three equivalent Se(16) atoms to form a mixture of corner and edge-sharing CuSe4 tetrahedra. The Cu(18)-Se(17) bond length is 2.42 Å. All Cu(18)-Se(16) bond lengths are 2.50 Å. In the seventeenth Cu site, Cu(19) is bonded to one Se(15) and three equivalent Se(16) atoms to form a mixture of corner and edge-sharing CuSe4 tetrahedra. The Cu(19)-Se(15) bond length is 2.36 Å. All Cu(19)-Se(16) bond lengths are 2.45 Å. In the eighteenth Cu site, Cu(20) is bonded to one Se(16) and three equivalent Se(17) atoms to form a mixture of corner and edge-sharing CuSe4 tetrahedra. The Cu(20)-Se(16) bond length is 2.57 Å. All Cu(20)-Se(17) bond lengths are 2.45 Å. In the nineteenth Cu site, Cu(21) is bonded to one Se(19) and three equivalent Se(18) atoms to form a mixture of corner and edge-sharing CuSe4 tetrahedra. The Cu(21)-Se(19) bond length is 2.47 Å. All Cu(21)-Se(18) bond lengths are 2.48 Å. In the twentieth Cu site, Cu(22) is bonded to one Se(17) and three equivalent Se(18) atoms to form a mixture of corner and edge-sharing CuSe4 tetrahedra. The Cu(22)-Se(17) bond length is 2.36 Å. All Cu(22)-Se(18) bond lengths are 2.45 Å. In the twenty-first Cu site, Cu(23) is bonded to one Se(20) and three equivalent Se(19) atoms to form a mixture of corner and edge-sharing CuSe4 tetrahedra. The Cu(23)-Se(20) bond length is 2.45 Å. All Cu(23)-Se(19) bond lengths are 2.46 Å. In the twenty-second Cu site, Cu(24) is bonded to one Se(1) and three equivalent Se(20) atoms to form a mixture of corner and edge-sharing CuSe4 tetrahedra. The Cu(24)-Se(1) bond length is 2.40 Å. All Cu(24)-Se(20) bond lengths are 2.48 Å. In the twenty-third Cu site, Cu(25) is bonded to one Se(18) and three equivalent Se(19) atoms to form a mixture of corner and edge-sharing CuSe4 tetrahedra. The Cu(25)-Se(18) bond length is 2.44 Å. All Cu(25)-Se(19) bond lengths are 2.49 Å. In the twenty-fourth Cu site, Cu(26) is bonded to one Se(19) and three equivalent Se(20) atoms to form a mixture of corner and edge-sharing CuSe4 tetrahedra. The Cu(26)-Se(19) bond length is 2.44 Å. All Cu(26)-Se(20) bond lengths are 2.46 Å. In the twenty-fifth Cu site, Cu(27) is bonded to one Se(20) and three equivalent Se(1) atoms to form a mixture of corner and edge-sharing CuSe4 tetrahedra. The Cu(27)-Se(20) bond length is 2.57 Å. All Cu(27)-Se(1) bond lengths are 2.42 Å. There are eighteen inequivalent Se sites. In the first Se site, Se(1) is bonded in a distorted pentagonal planar geometry to one Cu(2), one Cu(24), and three equivalent Cu(27) atoms. In the second Se site, Se(2) is bonded in a 7-coordinate geometry to one Cu(4), three equivalent Cu(1), and three equivalent Cu(2) atoms. In the third Se site, Se(3) is bonded in a distorted pentagonal planar geometry to one Cu(1), one Cu(5), and three equivalent Cu(4) atoms. In the fourth Se site, Se(4) is bonded in a 7-coordinate geometry to one Cu(6), three equivalent Cu(3), and three equivalent Cu(5) atoms. In the fifth Se site, Se(5) is bonded in a distorted pentagonal planar geometry to one Cu(3), one Cu(7), and three equivalent Cu(6) atoms. In the sixth Se site, Se(6) is bonded to one Cu(8) and three equivalent Cu(7) atoms to form corner-sharing SeCu4 tetrahedra. In the seventh Se site, Se(7) is bonded to one Cu(9) and three equivalent Cu(8) atoms to form corner-sharing SeCu4 tetrahedra. In the eighth Se site, Se(8,9,10) is bonded to one Cu(10,11,12) and three equivalent Cu(9) atoms to form corner-sharing SeCu4 tetrahedra. In the ninth Se site, Se(11) is bonded to one Cu(13) and three equivalent Cu(10,11,12) atoms to form corner-sharing SeCu4 tetrahedra. All Se(11)-Cu(10,11,12) bond lengths are 2.46 Å. In the tenth Se site, Se(12) is bonded to one Cu(15) and three equivalent Cu(13) atoms to form corner-sharing SeCu4 tetrahedra. In the eleventh Se site, Se(13) is bonded to one Cu(16) and three equivalent Cu(15) atoms to form corner-sharing SeCu4 tetrahedra. In the twelfth Se site, Se(14) is bonded in a 7-coordinate geometry to one Cu(17), three equivalent Cu(14), and three equivalent Cu(16) atoms. In the thirteenth Se site, Se(15) is bonded in a distorted pentagonal planar geometry to one Cu(14), one Cu(19), and three equivalent Cu(17) atoms. In the fourteenth Se site, Se(16) is bonded in a 7-coordinate geometry to one Cu(20), three equivalent Cu(18), and three equivalent Cu(19) atoms. In the fifteenth Se site, Se(17) is bonded in a distorted pentagonal planar geometry to one Cu(18), one Cu(22), and three equivalent Cu(20) atoms. In the sixteenth Se site, Se(18) is bonded to one Cu(25), three equivalent Cu(21), and three equivalent Cu(22) atoms to form distorted edge-sharing SeCu7 hexagonal pyramids. In the seventeenth Se site, Se(19) is bonded in a body-centered cubic geometry to one Cu(21), one Cu(26), three equivalent Cu(23), and three equivalent Cu(25) atoms. In the eighteenth Se site, Se(20) is bonded in a body-centered cubic geometry to one Cu(23), one Cu(27), three equivalent Cu(24), and three equivalent Cu(26) atoms.