The compound described by Ba4Cu2Si21 crystallizes in the orthorhombic Ama2 space group. There are four inequivalent Ba sites. In the first Ba site, Ba(1) is bonded in a 20-coordinate geometry to one Si(3), one Si(6), one Si(7), one Si(8), two equivalent Si(1), two equivalent Si(10), two equivalent Si(11), two equivalent Si(12), two equivalent Si(2), two equivalent Si(4), two equivalent Si(5), and two equivalent Si(9) atoms. In the second Ba site, Ba(2) is bonded in a 8-coordinate geometry to one Cu(2), two equivalent Cu(1), two equivalent Si(1), two equivalent Si(2), two equivalent Si(4), and two equivalent Si(5) atoms. In the third Ba site, Ba(3) is bonded in a 8-coordinate geometry to two equivalent Cu(2), two equivalent Si(3), two equivalent Si(6), two equivalent Si(7), and two equivalent Si(8) atoms. In the fourth Ba site, Ba(4) is bonded in a 23-coordinate geometry to one Cu(2), two equivalent Cu(1), one Si(13), one Si(6), two equivalent Si(1), two equivalent Si(11), two equivalent Si(12), two equivalent Si(2), two equivalent Si(5), four equivalent Si(10), and four equivalent Si(4) atoms. There are two inequivalent Cu sites. In the first Cu site, Cu(1) is bonded to two equivalent Ba(2), two equivalent Ba(4), two equivalent Si(1), and two equivalent Si(4) atoms to form distorted CuBa4Si4 tetrahedra that share corners with two equivalent Si(13)BaSi4 tetrahedra, corners with four equivalent Cu(2)Ba4Si4 tetrahedra, and edges with two equivalent Cu(1)Ba4Si4 tetrahedra. In the second Cu site, Cu(2) is bonded to one Ba(2), one Ba(4), two equivalent Ba(3), one Si(6), one Si(7), and two equivalent Si(5) atoms to form distorted CuBa4Si4 tetrahedra that share  a cornercorner with one Si(13)BaSi4 tetrahedra, corners with two equivalent Cu(2)Ba4Si4 tetrahedra, and corners with four equivalent Cu(1)Ba4Si4 tetrahedra. There are thirteen inequivalent Si sites. In the first Si site, Si(1) is bonded in a 7-coordinate geometry to one Ba(1), one Ba(2), one Ba(4), one Cu(1), one Si(1), one Si(12), and one Si(9) atom. In the second Si site, Si(2) is bonded in a 7-coordinate geometry to one Ba(1), one Ba(2), one Ba(4), one Si(11), one Si(13), one Si(5), and one Si(9) atom. In the third Si site, Si(3) is bonded in a 7-coordinate geometry to one Ba(1), two equivalent Ba(3), one Si(13), one Si(7), and two equivalent Si(9) atoms. In the fourth Si site, Si(4) is bonded in a 7-coordinate geometry to one Ba(1), one Ba(2), two equivalent Ba(4), one Cu(1), one Si(10), one Si(11), and one Si(4) atom. In the fifth Si site, Si(5) is bonded in a 7-coordinate geometry to one Ba(1), one Ba(2), one Ba(4), one Cu(2), one Si(10), one Si(12), and one Si(2) atom. In the sixth Si site, Si(6) is bonded in a 6-coordinate geometry to one Ba(1), one Ba(4), two equivalent Ba(3), one Cu(2), one Si(8), and two equivalent Si(10) atoms. In the seventh Si site, Si(7) is bonded in a 6-coordinate geometry to one Ba(1), two equivalent Ba(3), one Cu(2), one Si(3), and two equivalent Si(11) atoms. In the eighth Si site, Si(8) is bonded in a 7-coordinate geometry to one Ba(1), two equivalent Ba(3), one Si(13), one Si(6), and two equivalent Si(12) atoms. In the ninth Si site, Si(9) is bonded in a 5-coordinate geometry to one Ba(1), one Si(1), one Si(2), one Si(3), and one Si(9) atom. In the tenth Si site, Si(10) is bonded in a 5-coordinate geometry to one Ba(1), two equivalent Ba(4), one Si(10), one Si(4), one Si(5), and one Si(6) atom. In the eleventh Si site, Si(11) is bonded in a 5-coordinate geometry to one Ba(1), one Ba(4), one Si(12), one Si(2), one Si(4), and one Si(7) atom. In the twelfth Si site, Si(12) is bonded in a 5-coordinate geometry to one Ba(1), one Ba(4), one Si(1), one Si(11), one Si(5), and one Si(8) atom. In the thirteenth Si site, Si(13) is bonded to one Ba(4), one Si(3), one Si(8), and two equivalent Si(2) atoms to form distorted SiBaSi4 tetrahedra that share  a cornercorner with one Cu(2)Ba4Si4 tetrahedra and corners with two equivalent Cu(1)Ba4Si4 tetrahedra. is represented by the CIF file [CIF]
data_Ba4Cu2Si21
_symmetry_space_group_name_H-M   'P 1'
_cell_length_a   10.407
_cell_length_b   10.407
_cell_length_c   10.372
_cell_angle_alpha   90.000
_cell_angle_beta   90.000
_cell_angle_gamma   90.044
_symmetry_Int_Tables_number   1
_cell_volume   1123.239
_cell_formula_units_Z   2
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
  Ba  Ba0  1  0.498  0.002  0.250  1.0
  Ba  Ba1  1  0.002  0.498  0.750  1.0
  Ba  Ba2  1  0.742  0.491  0.250  1.0
  Ba  Ba3  1  0.491  0.742  0.750  1.0
  Ba  Ba4  1  0.010  0.010  0.500  1.0
  Ba  Ba5  1  0.259  0.490  0.250  1.0
  Ba  Ba6  1  0.490  0.259  0.750  1.0
  Ba  Ba7  1  0.010  0.010  0.000  1.0
  Cu  Cu8  1  0.500  0.500  0.500  1.0
  Cu  Cu9  1  0.250  0.001  0.750  1.0
  Cu  Cu10  1  0.001  0.250  0.250  1.0
  Cu  Cu11  1  0.500  0.500  0.000  1.0
  Si  Si12  1  0.501  0.688  0.369  1.0
  Si  Si13  1  0.616  1.000  0.556  1.0
  Si  Si14  1  0.807  0.884  0.250  1.0
  Si  Si15  1  0.501  0.310  0.129  1.0
  Si  Si16  1  0.378  1.000  0.937  1.0
  Si  Si17  1  0.188  0.124  0.250  1.0
  Si  Si18  1  0.501  0.310  0.371  1.0
  Si  Si19  1  0.378  1.000  0.563  1.0
  Si  Si20  1  0.501  0.688  0.131  1.0
  Si  Si21  1  0.616  1.000  0.944  1.0
  Si  Si22  1  0.814  0.123  0.250  1.0
  Si  Si23  1  0.194  0.882  0.250  1.0
  Si  Si24  1  0.688  0.501  0.869  1.0
  Si  Si25  1  1.000  0.616  0.056  1.0
  Si  Si26  1  0.884  0.807  0.750  1.0
  Si  Si27  1  0.310  0.501  0.629  1.0
  Si  Si28  1  1.000  0.378  0.437  1.0
  Si  Si29  1  0.124  0.188  0.750  1.0
  Si  Si30  1  0.310  0.501  0.871  1.0
  Si  Si31  1  1.000  0.378  0.063  1.0
  Si  Si32  1  0.688  0.501  0.631  1.0
  Si  Si33  1  1.000  0.616  0.444  1.0
  Si  Si34  1  0.123  0.814  0.750  1.0
  Si  Si35  1  0.882  0.194  0.750  1.0
  Si  Si36  1  0.684  0.814  0.433  1.0
  Si  Si37  1  0.316  0.184  0.066  1.0
  Si  Si38  1  0.316  0.184  0.434  1.0
  Si  Si39  1  0.684  0.814  0.067  1.0
  Si  Si40  1  0.686  0.185  0.067  1.0
  Si  Si41  1  0.317  0.815  0.435  1.0
  Si  Si42  1  0.317  0.815  0.065  1.0
  Si  Si43  1  0.686  0.185  0.433  1.0
  Si  Si44  1  0.814  0.684  0.933  1.0
  Si  Si45  1  0.184  0.316  0.566  1.0
  Si  Si46  1  0.184  0.316  0.934  1.0
  Si  Si47  1  0.814  0.684  0.567  1.0
  Si  Si48  1  0.185  0.686  0.567  1.0
  Si  Si49  1  0.815  0.317  0.935  1.0
  Si  Si50  1  0.815  0.317  0.565  1.0
  Si  Si51  1  0.185  0.686  0.933  1.0
  Si  Si52  1  0.747  0.001  0.750  1.0
  Si  Si53  1  0.001  0.747  0.250  1.0
[/CIF]
.
The compound described by Ba4Cu2Si21 crystallizes in the orthorhombic Ama2 space group. There are four inequivalent Ba sites. In the first Ba site, Ba(1) is bonded in a 20-coordinate geometry to one Si(3), one Si(6), one Si(7), one Si(8), two equivalent Si(1), two equivalent Si(10), two equivalent Si(11), two equivalent Si(12), two equivalent Si(2), two equivalent Si(4), two equivalent Si(5), and two equivalent Si(9) atoms. In the second Ba site, Ba(2) is bonded in a 8-coordinate geometry to one Cu(2), two equivalent Cu(1), two equivalent Si(1), two equivalent Si(2), two equivalent Si(4), and two equivalent Si(5) atoms. In the third Ba site, Ba(3) is bonded in a 8-coordinate geometry to two equivalent Cu(2), two equivalent Si(3), two equivalent Si(6), two equivalent Si(7), and two equivalent Si(8) atoms. In the fourth Ba site, Ba(4) is bonded in a 23-coordinate geometry to one Cu(2), two equivalent Cu(1), one Si(13), one Si(6), two equivalent Si(1), two equivalent Si(11), two equivalent Si(12), two equivalent Si(2), two equivalent Si(5), four equivalent Si(10), and four equivalent Si(4) atoms. There are two inequivalent Cu sites. In the first Cu site, Cu(1) is bonded to two equivalent Ba(2), two equivalent Ba(4), two equivalent Si(1), and two equivalent Si(4) atoms to form distorted CuBa4Si4 tetrahedra that share corners with two equivalent Si(13)BaSi4 tetrahedra, corners with four equivalent Cu(2)Ba4Si4 tetrahedra, and edges with two equivalent Cu(1)Ba4Si4 tetrahedra. In the second Cu site, Cu(2) is bonded to one Ba(2), one Ba(4), two equivalent Ba(3), one Si(6), one Si(7), and two equivalent Si(5) atoms to form distorted CuBa4Si4 tetrahedra that share  a cornercorner with one Si(13)BaSi4 tetrahedra, corners with two equivalent Cu(2)Ba4Si4 tetrahedra, and corners with four equivalent Cu(1)Ba4Si4 tetrahedra. There are thirteen inequivalent Si sites. In the first Si site, Si(1) is bonded in a 7-coordinate geometry to one Ba(1), one Ba(2), one Ba(4), one Cu(1), one Si(1), one Si(12), and one Si(9) atom. In the second Si site, Si(2) is bonded in a 7-coordinate geometry to one Ba(1), one Ba(2), one Ba(4), one Si(11), one Si(13), one Si(5), and one Si(9) atom. In the third Si site, Si(3) is bonded in a 7-coordinate geometry to one Ba(1), two equivalent Ba(3), one Si(13), one Si(7), and two equivalent Si(9) atoms. In the fourth Si site, Si(4) is bonded in a 7-coordinate geometry to one Ba(1), one Ba(2), two equivalent Ba(4), one Cu(1), one Si(10), one Si(11), and one Si(4) atom. In the fifth Si site, Si(5) is bonded in a 7-coordinate geometry to one Ba(1), one Ba(2), one Ba(4), one Cu(2), one Si(10), one Si(12), and one Si(2) atom. In the sixth Si site, Si(6) is bonded in a 6-coordinate geometry to one Ba(1), one Ba(4), two equivalent Ba(3), one Cu(2), one Si(8), and two equivalent Si(10) atoms. In the seventh Si site, Si(7) is bonded in a 6-coordinate geometry to one Ba(1), two equivalent Ba(3), one Cu(2), one Si(3), and two equivalent Si(11) atoms. In the eighth Si site, Si(8) is bonded in a 7-coordinate geometry to one Ba(1), two equivalent Ba(3), one Si(13), one Si(6), and two equivalent Si(12) atoms. In the ninth Si site, Si(9) is bonded in a 5-coordinate geometry to one Ba(1), one Si(1), one Si(2), one Si(3), and one Si(9) atom. In the tenth Si site, Si(10) is bonded in a 5-coordinate geometry to one Ba(1), two equivalent Ba(4), one Si(10), one Si(4), one Si(5), and one Si(6) atom. In the eleventh Si site, Si(11) is bonded in a 5-coordinate geometry to one Ba(1), one Ba(4), one Si(12), one Si(2), one Si(4), and one Si(7) atom. In the twelfth Si site, Si(12) is bonded in a 5-coordinate geometry to one Ba(1), one Ba(4), one Si(1), one Si(11), one Si(5), and one Si(8) atom. In the thirteenth Si site, Si(13) is bonded to one Ba(4), one Si(3), one Si(8), and two equivalent Si(2) atoms to form distorted SiBaSi4 tetrahedra that share  a cornercorner with one Cu(2)Ba4Si4 tetrahedra and corners with two equivalent Cu(1)Ba4Si4 tetrahedra. is represented by the CIF file [CIF]
data_Ba4Cu2Si21
_symmetry_space_group_name_H-M   'P 1'
_cell_length_a   10.407
_cell_length_b   10.407
_cell_length_c   10.372
_cell_angle_alpha   90.000
_cell_angle_beta   90.000
_cell_angle_gamma   90.044
_symmetry_Int_Tables_number   1
_cell_volume   1123.239
_cell_formula_units_Z   2
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
  Ba  Ba0  1  0.498  0.002  0.250  1.0
  Ba  Ba1  1  0.002  0.498  0.750  1.0
  Ba  Ba2  1  0.742  0.491  0.250  1.0
  Ba  Ba3  1  0.491  0.742  0.750  1.0
  Ba  Ba4  1  0.010  0.010  0.500  1.0
  Ba  Ba5  1  0.259  0.490  0.250  1.0
  Ba  Ba6  1  0.490  0.259  0.750  1.0
  Ba  Ba7  1  0.010  0.010  0.000  1.0
  Cu  Cu8  1  0.500  0.500  0.500  1.0
  Cu  Cu9  1  0.250  0.001  0.750  1.0
  Cu  Cu10  1  0.001  0.250  0.250  1.0
  Cu  Cu11  1  0.500  0.500  0.000  1.0
  Si  Si12  1  0.501  0.688  0.369  1.0
  Si  Si13  1  0.616  1.000  0.556  1.0
  Si  Si14  1  0.807  0.884  0.250  1.0
  Si  Si15  1  0.501  0.310  0.129  1.0
  Si  Si16  1  0.378  1.000  0.937  1.0
  Si  Si17  1  0.188  0.124  0.250  1.0
  Si  Si18  1  0.501  0.310  0.371  1.0
  Si  Si19  1  0.378  1.000  0.563  1.0
  Si  Si20  1  0.501  0.688  0.131  1.0
  Si  Si21  1  0.616  1.000  0.944  1.0
  Si  Si22  1  0.814  0.123  0.250  1.0
  Si  Si23  1  0.194  0.882  0.250  1.0
  Si  Si24  1  0.688  0.501  0.869  1.0
  Si  Si25  1  1.000  0.616  0.056  1.0
  Si  Si26  1  0.884  0.807  0.750  1.0
  Si  Si27  1  0.310  0.501  0.629  1.0
  Si  Si28  1  1.000  0.378  0.437  1.0
  Si  Si29  1  0.124  0.188  0.750  1.0
  Si  Si30  1  0.310  0.501  0.871  1.0
  Si  Si31  1  1.000  0.378  0.063  1.0
  Si  Si32  1  0.688  0.501  0.631  1.0
  Si  Si33  1  1.000  0.616  0.444  1.0
  Si  Si34  1  0.123  0.814  0.750  1.0
  Si  Si35  1  0.882  0.194  0.750  1.0
  Si  Si36  1  0.684  0.814  0.433  1.0
  Si  Si37  1  0.316  0.184  0.066  1.0
  Si  Si38  1  0.316  0.184  0.434  1.0
  Si  Si39  1  0.684  0.814  0.067  1.0
  Si  Si40  1  0.686  0.185  0.067  1.0
  Si  Si41  1  0.317  0.815  0.435  1.0
  Si  Si42  1  0.317  0.815  0.065  1.0
  Si  Si43  1  0.686  0.185  0.433  1.0
  Si  Si44  1  0.814  0.684  0.933  1.0
  Si  Si45  1  0.184  0.316  0.566  1.0
  Si  Si46  1  0.184  0.316  0.934  1.0
  Si  Si47  1  0.814  0.684  0.567  1.0
  Si  Si48  1  0.185  0.686  0.567  1.0
  Si  Si49  1  0.815  0.317  0.935  1.0
  Si  Si50  1  0.815  0.317  0.565  1.0
  Si  Si51  1  0.185  0.686  0.933  1.0
  Si  Si52  1  0.747  0.001  0.750  1.0
  Si  Si53  1  0.001  0.747  0.250  1.0
[/CIF]
.