The structure described by MgU8Se17 crystallizes in the monoclinic C2/m space group. Mg(1) is bonded in an octahedral geometry to two equivalent Se(4) and four equivalent Se(2) atoms. There are three inequivalent U sites. In the first U site, U(1) is bonded in a 8-coordinate geometry to one Se(2), one Se(3), one Se(4), one Se(5), one Se(6), one Se(7), and two equivalent Se(1) atoms. In the second U site, U(2) is bonded in a 8-coordinate geometry to one Se(3), one Se(4), one Se(5), one Se(7), and four equivalent Se(1) atoms. In the third U site, U(3) is bonded in a 8-coordinate geometry to one Se(3), one Se(5), two equivalent Se(6), and four equivalent Se(2) atoms. There are seven inequivalent Se sites. In the first Se site, Se(6) is bonded to two equivalent U(1) and two equivalent U(3) atoms to form SeU4 trigonal pyramids that share corners with two equivalent Se(7)U6 octahedra, corners with six equivalent Se(2)MgU3 tetrahedra, corners with two equivalent Se(4)MgU3 trigonal pyramids, edges with two equivalent Se(2)MgU3 tetrahedra, and  an edgeedge with one Se(6)U4 trigonal pyramid. The corner-sharing octahedral tilt angles are 37°. In the second Se site, Se(7) is bonded to two equivalent U(2) and four equivalent U(1) atoms to form SeU6 octahedra that share corners with four equivalent Se(2)MgU3 tetrahedra, corners with four equivalent Se(6)U4 trigonal pyramids, and faces with two equivalent Se(4)MgU3 trigonal pyramids. In the third Se site, Se(1) is bonded in a rectangular see-saw-like geometry to two equivalent U(1) and two equivalent U(2) atoms. In the fourth Se site, Se(2) is bonded to one Mg(1), one U(1), and two equivalent U(3) atoms to form distorted SeMgU3 tetrahedra that share  a cornercorner with one Se(7)U6 octahedra, corners with five equivalent Se(2)MgU3 tetrahedra,  a cornercorner with one Se(4)MgU3 trigonal pyramid, corners with three equivalent Se(6)U4 trigonal pyramids, edges with two equivalent Se(2)MgU3 tetrahedra,  an edgeedge with one Se(4)MgU3 trigonal pyramid, and  an edgeedge with one Se(6)U4 trigonal pyramid. The corner-sharing octahedral tilt angles are 53°. In the fifth Se site, Se(3) is bonded in a rectangular see-saw-like geometry to one U(2), one U(3), and two equivalent U(1) atoms. In the sixth Se site, Se(4) is bonded to one Mg(1), one U(2), and two equivalent U(1) atoms to form distorted SeMgU3 trigonal pyramids that share corners with two equivalent Se(2)MgU3 tetrahedra,  a cornercorner with one Se(4)MgU3 trigonal pyramid, corners with two equivalent Se(6)U4 trigonal pyramids, edges with two equivalent Se(2)MgU3 tetrahedra, and  a faceface with one Se(7)U6 octahedra. In the seventh Se site, Se(5) is bonded in a rectangular see-saw-like geometry to one U(2), one U(3), and two equivalent U(1) atoms. is represented by the CIF card [CIF]
data_MgU8Se17
_symmetry_space_group_name_H-M   'P 1'
_cell_length_a   8.330
_cell_length_b   8.330
_cell_length_c   11.069
_cell_angle_alpha   79.886
_cell_angle_beta   79.886
_cell_angle_gamma   64.032
_symmetry_Int_Tables_number   1
_cell_volume   675.633
_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
  Mg  Mg0  1  0.000  0.000  0.000  1.0
  U  U1  1  0.185  0.698  0.702  1.0
  U  U2  1  0.698  0.185  0.702  1.0
  U  U3  1  0.815  0.302  0.298  1.0
  U  U4  1  0.302  0.815  0.298  1.0
  U  U5  1  0.796  0.796  0.458  1.0
  U  U6  1  0.204  0.204  0.542  1.0
  U  U7  1  0.319  0.319  0.016  1.0
  U  U8  1  0.681  0.681  0.984  1.0
  Se  Se9  1  0.177  0.568  0.467  1.0
  Se  Se10  1  0.568  0.177  0.467  1.0
  Se  Se11  1  0.823  0.432  0.533  1.0
  Se  Se12  1  0.432  0.823  0.533  1.0
  Se  Se13  1  0.057  0.670  0.966  1.0
  Se  Se14  1  0.670  0.057  0.966  1.0
  Se  Se15  1  0.943  0.330  0.034  1.0
  Se  Se16  1  0.330  0.943  0.034  1.0
  Se  Se17  1  0.790  0.790  0.721  1.0
  Se  Se18  1  0.210  0.210  0.279  1.0
  Se  Se19  1  0.941  0.941  0.233  1.0
  Se  Se20  1  0.059  0.059  0.767  1.0
  Se  Se21  1  0.697  0.697  0.249  1.0
  Se  Se22  1  0.303  0.303  0.751  1.0
  Se  Se23  1  0.480  0.480  0.170  1.0
  Se  Se24  1  0.520  0.520  0.830  1.0
  Se  Se25  1  0.000  0.000  0.500  1.0
[/CIF]
.
The structure described by MgU8Se17 crystallizes in the monoclinic C2/m space group. Mg(1) is bonded in an octahedral geometry to two equivalent Se(4) and four equivalent Se(2) atoms. There are three inequivalent U sites. In the first U site, U(1) is bonded in a 8-coordinate geometry to one Se(2), one Se(3), one Se(4), one Se(5), one Se(6), one Se(7), and two equivalent Se(1) atoms. In the second U site, U(2) is bonded in a 8-coordinate geometry to one Se(3), one Se(4), one Se(5), one Se(7), and four equivalent Se(1) atoms. In the third U site, U(3) is bonded in a 8-coordinate geometry to one Se(3), one Se(5), two equivalent Se(6), and four equivalent Se(2) atoms. There are seven inequivalent Se sites. In the first Se site, Se(6) is bonded to two equivalent U(1) and two equivalent U(3) atoms to form SeU4 trigonal pyramids that share corners with two equivalent Se(7)U6 octahedra, corners with six equivalent Se(2)MgU3 tetrahedra, corners with two equivalent Se(4)MgU3 trigonal pyramids, edges with two equivalent Se(2)MgU3 tetrahedra, and  an edgeedge with one Se(6)U4 trigonal pyramid. The corner-sharing octahedral tilt angles are 37°. In the second Se site, Se(7) is bonded to two equivalent U(2) and four equivalent U(1) atoms to form SeU6 octahedra that share corners with four equivalent Se(2)MgU3 tetrahedra, corners with four equivalent Se(6)U4 trigonal pyramids, and faces with two equivalent Se(4)MgU3 trigonal pyramids. In the third Se site, Se(1) is bonded in a rectangular see-saw-like geometry to two equivalent U(1) and two equivalent U(2) atoms. In the fourth Se site, Se(2) is bonded to one Mg(1), one U(1), and two equivalent U(3) atoms to form distorted SeMgU3 tetrahedra that share  a cornercorner with one Se(7)U6 octahedra, corners with five equivalent Se(2)MgU3 tetrahedra,  a cornercorner with one Se(4)MgU3 trigonal pyramid, corners with three equivalent Se(6)U4 trigonal pyramids, edges with two equivalent Se(2)MgU3 tetrahedra,  an edgeedge with one Se(4)MgU3 trigonal pyramid, and  an edgeedge with one Se(6)U4 trigonal pyramid. The corner-sharing octahedral tilt angles are 53°. In the fifth Se site, Se(3) is bonded in a rectangular see-saw-like geometry to one U(2), one U(3), and two equivalent U(1) atoms. In the sixth Se site, Se(4) is bonded to one Mg(1), one U(2), and two equivalent U(1) atoms to form distorted SeMgU3 trigonal pyramids that share corners with two equivalent Se(2)MgU3 tetrahedra,  a cornercorner with one Se(4)MgU3 trigonal pyramid, corners with two equivalent Se(6)U4 trigonal pyramids, edges with two equivalent Se(2)MgU3 tetrahedra, and  a faceface with one Se(7)U6 octahedra. In the seventh Se site, Se(5) is bonded in a rectangular see-saw-like geometry to one U(2), one U(3), and two equivalent U(1) atoms. is represented by the CIF card [CIF]
data_MgU8Se17
_symmetry_space_group_name_H-M   'P 1'
_cell_length_a   8.330
_cell_length_b   8.330
_cell_length_c   11.069
_cell_angle_alpha   79.886
_cell_angle_beta   79.886
_cell_angle_gamma   64.032
_symmetry_Int_Tables_number   1
_cell_volume   675.633
_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
  Mg  Mg0  1  0.000  0.000  0.000  1.0
  U  U1  1  0.185  0.698  0.702  1.0
  U  U2  1  0.698  0.185  0.702  1.0
  U  U3  1  0.815  0.302  0.298  1.0
  U  U4  1  0.302  0.815  0.298  1.0
  U  U5  1  0.796  0.796  0.458  1.0
  U  U6  1  0.204  0.204  0.542  1.0
  U  U7  1  0.319  0.319  0.016  1.0
  U  U8  1  0.681  0.681  0.984  1.0
  Se  Se9  1  0.177  0.568  0.467  1.0
  Se  Se10  1  0.568  0.177  0.467  1.0
  Se  Se11  1  0.823  0.432  0.533  1.0
  Se  Se12  1  0.432  0.823  0.533  1.0
  Se  Se13  1  0.057  0.670  0.966  1.0
  Se  Se14  1  0.670  0.057  0.966  1.0
  Se  Se15  1  0.943  0.330  0.034  1.0
  Se  Se16  1  0.330  0.943  0.034  1.0
  Se  Se17  1  0.790  0.790  0.721  1.0
  Se  Se18  1  0.210  0.210  0.279  1.0
  Se  Se19  1  0.941  0.941  0.233  1.0
  Se  Se20  1  0.059  0.059  0.767  1.0
  Se  Se21  1  0.697  0.697  0.249  1.0
  Se  Se22  1  0.303  0.303  0.751  1.0
  Se  Se23  1  0.480  0.480  0.170  1.0
  Se  Se24  1  0.520  0.520  0.830  1.0
  Se  Se25  1  0.000  0.000  0.500  1.0
[/CIF]
.