data_Mg2CuSe4
_symmetry_space_group_name_H-M   'P 1'
_cell_length_a   5.89
_cell_length_b   5.89
_cell_length_c   5.89
_cell_angle_alpha   109.16
_cell_angle_beta   109.16
_cell_angle_gamma   110.09
_symmetry_Int_Tables_number   1
_chemical_formula_structural   Mg2CuSe4
_chemical_formula_sum   'Mg2 Cu1 Se4'
_cell_volume   157.16
_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  Mg1  1  0.41  0.0  0.69  1.0
  Mg  Mg2  1  0.0  0.41  0.69  1.0
  Cu  Cu0  1  0.0  0.0  0.0  1.0
  Se  Se3  1  0.25  0.75  0.37  1.0
  Se  Se4  1  0.75  0.25  0.37  1.0
  Se  Se5  1  0.75  0.75  0.37  1.0
  Se  Se6  1  0.66  0.66  0.0  1.0

data_Mg2CuSe4
_symmetry_space_group_name_H-M   Cm
_cell_length_a   6.75
_cell_length_b   9.65
_cell_length_c   5.89
_cell_angle_alpha   90.0
_cell_angle_beta   124.95
_cell_angle_gamma   90.0
_symmetry_Int_Tables_number   8
_chemical_formula_structural   Mg2CuSe4
_chemical_formula_sum   'Mg4 Cu2 Se8'
_cell_volume   314.32
_cell_formula_units_Z   2
loop_
 _symmetry_equiv_pos_site_id
 _symmetry_equiv_pos_as_xyz
  1  'x, y, z'
  2  'x, -y, z'
  3  'x+1/2, y+1/2, z'
  4  'x+1/2, -y+1/2, 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  4  0.02  0.29  0.31  1.0
  Cu  Cu1  2  0.0  0.0  0.0  1.0
  Se  Se2  4  0.13  0.25  0.63  1.0
  Se  Se3  2  0.16  0.5  0.0  1.0
  Se  Se4  2  0.38  0.0  0.63  1.0

Mg Mg Cu Se Se Se Se
Mg2CuSe4 crystallizes in the monoclinic Cm space group. Mg(1) is bonded in a single-bond geometry to one Se(1), one Se(2), and one Se(3) atom. The Mg(1)-Se(1) bond length is 1.60 Å. The Mg(1)-Se(2) bond length is 3.21 Å. The Mg(1)-Se(3) bond length is 3.21 Å. Cu(1) is bonded in a distorted single-bond geometry to one Se(2) and one Se(3) atom. The Cu(1)-Se(2) bond length is 3.06 Å. The Cu(1)-Se(3) bond length is 2.29 Å. There are three inequivalent Se sites. In the first Se site, Se(1) is bonded in a single-bond geometry to one Mg(1) and two equivalent Se(2) atoms. There is one shorter (2.92 Å) and one longer (2.97 Å) Se(1)-Se(2) bond length. In the second Se site, Se(2) is bonded in a 8-coordinate geometry to two equivalent Mg(1), one Cu(1), one Se(3), and four equivalent Se(1) atoms. The Se(2)-Se(3) bond length is 1.91 Å. In the third Se site, Se(3) is bonded in a distorted tetrahedral geometry to two equivalent Mg(1), one Cu(1), and one Se(2) atom.