data_MgAsSe2
_symmetry_space_group_name_H-M   'P 1'
_cell_length_a   6.84
_cell_length_b   6.84
_cell_length_c   6.84
_cell_angle_alpha   33.86
_cell_angle_beta   33.86
_cell_angle_gamma   33.86
_symmetry_Int_Tables_number   1
_chemical_formula_structural   MgAsSe2
_chemical_formula_sum   'Mg1 As1 Se2'
_cell_volume   88.56
_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  0.0  0.0  0.0  1.0
  As  As0  1  0.0  0.0  0.66  1.0
  Se  Se2  1  0.0  0.0  0.51  1.0
  Se  Se3  1  0.0  0.0  0.82  1.0

data_MgAsSe2
_symmetry_space_group_name_H-M   Cm
_cell_length_a   6.9
_cell_length_b   3.98
_cell_length_c   6.84
_cell_angle_alpha   90.0
_cell_angle_beta   109.65
_cell_angle_gamma   90.0
_symmetry_Int_Tables_number   8
_chemical_formula_structural   MgAsSe2
_chemical_formula_sum   'Mg2 As2 Se4'
_cell_volume   177.11
_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  2  0.0  0.0  0.0  1.0
  As  As1  2  0.0  0.0  0.34  1.0
  Se  Se2  2  0.0  0.0  0.18  1.0
  Se  Se3  2  0.0  0.0  0.49  1.0

Mg As Se Se
MgAsSe2 is Cyanogen Chloride-like structured and crystallizes in the monoclinic Cm space group. The structure is zero-dimensional and consists of two MgAsSe2 clusters. Mg(1) is bonded in a single-bond geometry to one Se(2) atom. The Mg(1)-Se(2) bond length is 1.25 Å. As(1) is bonded in a linear geometry to one Se(1) and one Se(2) atom. The As(1)-Se(1) bond length is 1.05 Å. The As(1)-Se(2) bond length is 1.05 Å. There are two inequivalent Se sites. In the first Se site, Se(1) is bonded in a single-bond geometry to one As(1) atom. In the second Se site, Se(2) is bonded in a linear geometry to one Mg(1) and one As(1) atom.