The crystal structure described by 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. is represented by the CIF file [CIF]
data_Cu27Se20
_symmetry_space_group_name_H-M   'P 1'
_cell_length_a   4.022
_cell_length_b   4.022
_cell_length_c   79.839
_cell_angle_alpha   59.749
_cell_angle_beta   59.749
_cell_angle_gamma   60.000
_symmetry_Int_Tables_number   1
_cell_volume   909.830
_cell_formula_units_Z   1
loop_
 _symmetry_equiv_pos_site_id
 _symmetry_equiv_pos_as_xyz
  1  'x, y, z'
loop_
 _atom_site_type_symbol
 _atom_site_label
 _atom_site_symmetry_multiplicity
 _atom_site_fract_x
 _atom_site_fract_y
 _atom_site_fract_z
 _atom_site_occupancy
  Cu  Cu0  1  0.246  0.246  0.063  1.0
  Cu  Cu1  1  0.755  0.755  0.037  1.0
  Cu  Cu2  1  0.245  0.245  0.163  1.0
  Cu  Cu3  1  0.745  0.745  0.088  1.0
  Cu  Cu4  1  0.755  0.755  0.137  1.0
  Cu  Cu5  1  0.744  0.744  0.188  1.0
  Cu  Cu6  1  0.742  0.742  0.239  1.0
  Cu  Cu7  1  0.750  0.750  0.288  1.0
  Cu  Cu8  1  0.750  0.750  0.338  1.0
  Cu  Cu9  1  0.749  0.749  0.388  1.0
  Cu  Cu10  1  0.750  0.750  0.438  1.0
  Cu  Cu11  1  0.750  0.750  0.488  1.0
  Cu  Cu12  1  0.749  0.749  0.538  1.0
  Cu  Cu13  1  0.246  0.246  0.663  1.0
  Cu  Cu14  1  0.750  0.750  0.588  1.0
  Cu  Cu15  1  0.763  0.763  0.636  1.0
  Cu  Cu16  1  0.744  0.744  0.688  1.0
  Cu  Cu17  1  0.246  0.246  0.763  1.0
  Cu  Cu18  1  0.755  0.755  0.737  1.0
  Cu  Cu19  1  0.744  0.744  0.788  1.0
  Cu  Cu20  1  0.254  0.254  0.862  1.0
  Cu  Cu21  1  0.754  0.754  0.837  1.0
  Cu  Cu22  1  0.250  0.250  0.913  1.0
  Cu  Cu23  1  0.244  0.244  0.963  1.0
  Cu  Cu24  1  0.758  0.758  0.886  1.0
  Cu  Cu25  1  0.749  0.749  0.938  1.0
  Cu  Cu26  1  0.735  0.735  0.990  1.0
  Se  Se27  1  0.998  0.998  0.000  1.0
  Se  Se28  1  0.008  0.008  0.049  1.0
  Se  Se29  1  0.997  0.997  0.100  1.0
  Se  Se30  1  0.007  0.007  0.149  1.0
  Se  Se31  1  0.994  0.994  0.201  1.0
  Se  Se32  1  0.001  0.001  0.250  1.0
  Se  Se33  1  1.000  1.000  0.300  1.0
  Se  Se34  1  0.000  0.000  0.350  1.0
  Se  Se35  1  0.001  0.001  0.400  1.0
  Se  Se36  1  0.000  0.000  0.450  1.0
  Se  Se37  1  0.000  0.000  0.500  1.0
  Se  Se38  1  0.001  0.001  0.550  1.0
  Se  Se39  1  0.005  0.005  0.599  1.0
  Se  Se40  1  0.007  0.007  0.649  1.0
  Se  Se41  1  0.997  0.997  0.700  1.0
  Se  Se42  1  0.008  0.008  0.749  1.0
  Se  Se43  1  0.997  0.997  0.800  1.0
  Se  Se44  1  0.009  0.009  0.849  1.0
  Se  Se45  1  0.000  0.000  0.900  1.0
  Se  Se46  1  0.999  0.999  0.950  1.0
[/CIF]
.
The crystal structure described by 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. is represented by the CIF file [CIF]
data_Cu27Se20
_symmetry_space_group_name_H-M   'P 1'
_cell_length_a   4.022
_cell_length_b   4.022
_cell_length_c   79.839
_cell_angle_alpha   59.749
_cell_angle_beta   59.749
_cell_angle_gamma   60.000
_symmetry_Int_Tables_number   1
_cell_volume   909.830
_cell_formula_units_Z   1
loop_
 _symmetry_equiv_pos_site_id
 _symmetry_equiv_pos_as_xyz
  1  'x, y, z'
loop_
 _atom_site_type_symbol
 _atom_site_label
 _atom_site_symmetry_multiplicity
 _atom_site_fract_x
 _atom_site_fract_y
 _atom_site_fract_z
 _atom_site_occupancy
  Cu  Cu0  1  0.246  0.246  0.063  1.0
  Cu  Cu1  1  0.755  0.755  0.037  1.0
  Cu  Cu2  1  0.245  0.245  0.163  1.0
  Cu  Cu3  1  0.745  0.745  0.088  1.0
  Cu  Cu4  1  0.755  0.755  0.137  1.0
  Cu  Cu5  1  0.744  0.744  0.188  1.0
  Cu  Cu6  1  0.742  0.742  0.239  1.0
  Cu  Cu7  1  0.750  0.750  0.288  1.0
  Cu  Cu8  1  0.750  0.750  0.338  1.0
  Cu  Cu9  1  0.749  0.749  0.388  1.0
  Cu  Cu10  1  0.750  0.750  0.438  1.0
  Cu  Cu11  1  0.750  0.750  0.488  1.0
  Cu  Cu12  1  0.749  0.749  0.538  1.0
  Cu  Cu13  1  0.246  0.246  0.663  1.0
  Cu  Cu14  1  0.750  0.750  0.588  1.0
  Cu  Cu15  1  0.763  0.763  0.636  1.0
  Cu  Cu16  1  0.744  0.744  0.688  1.0
  Cu  Cu17  1  0.246  0.246  0.763  1.0
  Cu  Cu18  1  0.755  0.755  0.737  1.0
  Cu  Cu19  1  0.744  0.744  0.788  1.0
  Cu  Cu20  1  0.254  0.254  0.862  1.0
  Cu  Cu21  1  0.754  0.754  0.837  1.0
  Cu  Cu22  1  0.250  0.250  0.913  1.0
  Cu  Cu23  1  0.244  0.244  0.963  1.0
  Cu  Cu24  1  0.758  0.758  0.886  1.0
  Cu  Cu25  1  0.749  0.749  0.938  1.0
  Cu  Cu26  1  0.735  0.735  0.990  1.0
  Se  Se27  1  0.998  0.998  0.000  1.0
  Se  Se28  1  0.008  0.008  0.049  1.0
  Se  Se29  1  0.997  0.997  0.100  1.0
  Se  Se30  1  0.007  0.007  0.149  1.0
  Se  Se31  1  0.994  0.994  0.201  1.0
  Se  Se32  1  0.001  0.001  0.250  1.0
  Se  Se33  1  1.000  1.000  0.300  1.0
  Se  Se34  1  0.000  0.000  0.350  1.0
  Se  Se35  1  0.001  0.001  0.400  1.0
  Se  Se36  1  0.000  0.000  0.450  1.0
  Se  Se37  1  0.000  0.000  0.500  1.0
  Se  Se38  1  0.001  0.001  0.550  1.0
  Se  Se39  1  0.005  0.005  0.599  1.0
  Se  Se40  1  0.007  0.007  0.649  1.0
  Se  Se41  1  0.997  0.997  0.700  1.0
  Se  Se42  1  0.008  0.008  0.749  1.0
  Se  Se43  1  0.997  0.997  0.800  1.0
  Se  Se44  1  0.009  0.009  0.849  1.0
  Se  Se45  1  0.000  0.000  0.900  1.0
  Se  Se46  1  0.999  0.999  0.950  1.0
[/CIF]
.