The crystal structure described by Fe2NiSe4 crystallizes in the monoclinic Cm space group. There are two inequivalent Fe sites. In the first Fe site, Fe(1) is bonded to one Se(1), one Se(2), two equivalent Se(3), and two equivalent Se(4) atoms to form FeSe6 octahedra that share corners with six equivalent Fe(2)Se6 octahedra, corners with six equivalent Ni(1)Se6 octahedra, edges with two equivalent Fe(1)Se6 octahedra,  a faceface with one Fe(2)Se6 octahedra, and  a faceface with one Ni(1)Se6 octahedra. The corner-sharing octahedral tilt angles range from 50-53°. In the second Fe site, Fe(2) is bonded to one Se(2), one Se(4), two equivalent Se(1), and two equivalent Se(3) atoms to form FeSe6 octahedra that share corners with six equivalent Fe(1)Se6 octahedra, edges with two equivalent Fe(2)Se6 octahedra, edges with four equivalent Ni(1)Se6 octahedra, and  a faceface with one Fe(1)Se6 octahedra. The corner-sharing octahedral tilt angles range from 50-53°. Ni(1) is bonded to one Se(1), one Se(3), two equivalent Se(2), and two equivalent Se(4) atoms to form distorted NiSe6 octahedra that share corners with six equivalent Fe(1)Se6 octahedra, edges with two equivalent Ni(1)Se6 octahedra, edges with four equivalent Fe(2)Se6 octahedra, and  a faceface with one Fe(1)Se6 octahedra. The corner-sharing octahedral tilt angles range from 52-53°. There are four inequivalent Se sites. In the first Se site, Se(1) is bonded in a rectangular see-saw-like geometry to one Fe(1), two equivalent Fe(2), and one Ni(1) atom. In the second Se site, Se(2) is bonded in a rectangular see-saw-like geometry to one Fe(1), one Fe(2), and two equivalent Ni(1) atoms. In the third Se site, Se(3) is bonded in a 5-coordinate geometry to two equivalent Fe(1), two equivalent Fe(2), and one Ni(1) atom. In the fourth Se site, Se(4) is bonded in a 5-coordinate geometry to one Fe(2), two equivalent Fe(1), and two equivalent Ni(1) atoms. is represented by the CIF file [CIF]
data_Fe2NiSe4
_symmetry_space_group_name_H-M   'P 1'
_cell_length_a   6.289
_cell_length_b   6.289
_cell_length_c   6.197
_cell_angle_alpha   61.473
_cell_angle_beta   61.473
_cell_angle_gamma   32.973
_symmetry_Int_Tables_number   1
_cell_volume   115.676
_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
  Fe  Fe0  1  0.998  0.998  0.999  1.0
  Fe  Fe1  1  0.260  0.260  0.719  1.0
  Ni  Ni2  1  0.739  0.739  0.285  1.0
  Se  Se3  1  0.879  0.879  0.459  1.0
  Se  Se4  1  0.120  0.120  0.550  1.0
  Se  Se5  1  0.637  0.637  0.020  1.0
  Se  Se6  1  0.367  0.367  0.967  1.0
[/CIF]
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The crystal structure described by Fe2NiSe4 crystallizes in the monoclinic Cm space group. There are two inequivalent Fe sites. In the first Fe site, Fe(1) is bonded to one Se(1), one Se(2), two equivalent Se(3), and two equivalent Se(4) atoms to form FeSe6 octahedra that share corners with six equivalent Fe(2)Se6 octahedra, corners with six equivalent Ni(1)Se6 octahedra, edges with two equivalent Fe(1)Se6 octahedra,  a faceface with one Fe(2)Se6 octahedra, and  a faceface with one Ni(1)Se6 octahedra. The corner-sharing octahedral tilt angles range from 50-53°. In the second Fe site, Fe(2) is bonded to one Se(2), one Se(4), two equivalent Se(1), and two equivalent Se(3) atoms to form FeSe6 octahedra that share corners with six equivalent Fe(1)Se6 octahedra, edges with two equivalent Fe(2)Se6 octahedra, edges with four equivalent Ni(1)Se6 octahedra, and  a faceface with one Fe(1)Se6 octahedra. The corner-sharing octahedral tilt angles range from 50-53°. Ni(1) is bonded to one Se(1), one Se(3), two equivalent Se(2), and two equivalent Se(4) atoms to form distorted NiSe6 octahedra that share corners with six equivalent Fe(1)Se6 octahedra, edges with two equivalent Ni(1)Se6 octahedra, edges with four equivalent Fe(2)Se6 octahedra, and  a faceface with one Fe(1)Se6 octahedra. The corner-sharing octahedral tilt angles range from 52-53°. There are four inequivalent Se sites. In the first Se site, Se(1) is bonded in a rectangular see-saw-like geometry to one Fe(1), two equivalent Fe(2), and one Ni(1) atom. In the second Se site, Se(2) is bonded in a rectangular see-saw-like geometry to one Fe(1), one Fe(2), and two equivalent Ni(1) atoms. In the third Se site, Se(3) is bonded in a 5-coordinate geometry to two equivalent Fe(1), two equivalent Fe(2), and one Ni(1) atom. In the fourth Se site, Se(4) is bonded in a 5-coordinate geometry to one Fe(2), two equivalent Fe(1), and two equivalent Ni(1) atoms. is represented by the CIF file [CIF]
data_Fe2NiSe4
_symmetry_space_group_name_H-M   'P 1'
_cell_length_a   6.289
_cell_length_b   6.289
_cell_length_c   6.197
_cell_angle_alpha   61.473
_cell_angle_beta   61.473
_cell_angle_gamma   32.973
_symmetry_Int_Tables_number   1
_cell_volume   115.676
_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
  Fe  Fe0  1  0.998  0.998  0.999  1.0
  Fe  Fe1  1  0.260  0.260  0.719  1.0
  Ni  Ni2  1  0.739  0.739  0.285  1.0
  Se  Se3  1  0.879  0.879  0.459  1.0
  Se  Se4  1  0.120  0.120  0.550  1.0
  Se  Se5  1  0.637  0.637  0.020  1.0
  Se  Se6  1  0.367  0.367  0.967  1.0
[/CIF]
.