data_Mg(PSe)2
_symmetry_space_group_name_H-M   'P 1'
_cell_length_a   5.25
_cell_length_b   5.25
_cell_length_c   5.25
_cell_angle_alpha   120.07
_cell_angle_beta   120.07
_cell_angle_gamma   89.87
_symmetry_Int_Tables_number   1
_chemical_formula_structural   Mg(PSe)2
_chemical_formula_sum   'Mg1 P2 Se2'
_cell_volume   102.16
_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  Mg0  1  0.0  0.0  0.0  1.0
  P  P1  1  0.0  0.0  0.84  1.0
  P  P2  1  0.0  0.0  0.59  1.0
  Se  Se3  1  0.0  0.62  0.86  1.0
  Se  Se4  1  0.62  0.0  0.86  1.0

data_Mg(PSe)2
_symmetry_space_group_name_H-M   Cm
_cell_length_a   7.42
_cell_length_b   7.42
_cell_length_c   5.25
_cell_angle_alpha   90.0
_cell_angle_beta   134.94
_cell_angle_gamma   90.0
_symmetry_Int_Tables_number   8
_chemical_formula_structural   Mg(PSe)2
_chemical_formula_sum   'Mg2 P4 Se4'
_cell_volume   204.32
_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  2  0.0  0.0  0.0  1.0
  P  P1  2  0.09  0.5  0.59  1.0
  P  P2  2  0.34  0.5  0.84  1.0
  Se  Se3  4  0.05  0.19  0.24  1.0

Mg P P Se Se
Mg P P Se Se 0 1 o o - 0 3 o - - 0 4 - o - 1 2 o o o 1 3 o - o 1 4 - o o 
Mg(PSe)2 crystallizes in the monoclinic Cm space group. Mg(1) is bonded in a 3-coordinate geometry to one P(1), one P(2), and two equivalent Se(1) atoms. The Mg(1)-P(1) bond length is 0.85 Å. The Mg(1)-P(2) bond length is 3.12 Å. Both Mg(1)-Se(1) bond lengths are 1.74 Å. There are two inequivalent P sites. In the first P site, P(1) is bonded in a single-bond geometry to one Mg(1) and one P(2) atom. The P(1)-P(2) bond length is 1.28 Å. In the second P site, P(2) is bonded in a 4-coordinate geometry to one Mg(1), one P(1), and two equivalent Se(1) atoms. Both P(2)-Se(1) bond lengths are 2.83 Å. Se(1) is bonded in a single-bond geometry to one Mg(1) and one P(2) atom.