data_La(MgSe)2
_symmetry_space_group_name_H-M   'P 1'
_cell_length_a   6.0
_cell_length_b   6.0
_cell_length_c   6.0
_cell_angle_alpha   130.76
_cell_angle_beta   130.76
_cell_angle_gamma   72.19
_symmetry_Int_Tables_number   1
_chemical_formula_structural   La(MgSe)2
_chemical_formula_sum   'La1 Mg2 Se2'
_cell_volume   121.43
_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
  La  La0  1  0.0  0.0  0.0  1.0
  Mg  Mg1  1  0.0  0.0  0.26  1.0
  Mg  Mg2  1  0.0  0.0  0.44  1.0
  Se  Se3  1  0.0  0.5  0.43  1.0
  Se  Se4  1  0.5  0.0  0.43  1.0

data_La(MgSe)2
_symmetry_space_group_name_H-M   Cm
_cell_length_a   7.07
_cell_length_b   7.07
_cell_length_c   6.0
_cell_angle_alpha   90.0
_cell_angle_beta   126.1
_cell_angle_gamma   90.0
_symmetry_Int_Tables_number   8
_chemical_formula_structural   La(MgSe)2
_chemical_formula_sum   'La2 Mg4 Se4'
_cell_volume   242.86
_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
  La  La0  2  0.0  0.0  0.0  1.0
  Mg  Mg1  2  0.0  0.0  0.26  1.0
  Mg  Mg2  2  0.0  0.0  0.44  1.0
  Se  Se3  4  0.25  0.25  0.92  1.0

La Mg Mg Se Se
La(MgSe)2 crystallizes in the monoclinic Cm space group. There are two inequivalent Mg sites. In the first Mg site, Mg(1) is bonded in a tetrahedral geometry to one Mg(2), one La(1), and two equivalent Se(1) atoms. The Mg(1)-Mg(2) bond length is 1.10 Å. The Mg(1)-La(1) bond length is 1.56 Å. Both Mg(1)-Se(1) bond lengths are 2.48 Å. In the second Mg site, Mg(2) is bonded in a 3-coordinate geometry to one Mg(1) and two equivalent Se(1) atoms. Both Mg(2)-Se(1) bond lengths are 2.93 Å. La(1) is bonded in a 5-coordinate geometry to one Mg(1) and four equivalent Se(1) atoms. There are two shorter (2.36 Å) and two longer (2.71 Å) La(1)-Se(1) bond lengths. Se(1) is bonded in a 4-coordinate geometry to one Mg(1), one Mg(2), and two equivalent La(1) atoms.