data_Hg3Se
_symmetry_space_group_name_H-M   'P 1'
_cell_length_a   5.21
_cell_length_b   5.21
_cell_length_c   5.21
_cell_angle_alpha   60.0
_cell_angle_beta   60.0
_cell_angle_gamma   60.0
_symmetry_Int_Tables_number   1
_chemical_formula_structural   Hg3Se
_chemical_formula_sum   'Hg3 Se1'
_cell_volume   100.08
_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
  Hg  Hg0  1  0.84  0.84  0.84  1.0
  Hg  Hg1  1  0.68  0.68  0.68  1.0
  Hg  Hg2  1  0.53  0.53  0.53  1.0
  Se  Se3  1  0.0  0.0  0.0  1.0

data_Hg3Se
_symmetry_space_group_name_H-M   R3m
_cell_length_a   5.21
_cell_length_b   5.21
_cell_length_c   12.77
_cell_angle_alpha   90.0
_cell_angle_beta   90.0
_cell_angle_gamma   120.0
_symmetry_Int_Tables_number   160
_chemical_formula_structural   Hg3Se
_chemical_formula_sum   'Hg9 Se3'
_cell_volume   300.23
_cell_formula_units_Z   3
loop_
 _symmetry_equiv_pos_site_id
 _symmetry_equiv_pos_as_xyz
  1  'x, y, z'
  2  '-y, x-y, z'
  3  '-x+y, -x, z'
  4  '-y, -x, z'
  5  '-x+y, y, z'
  6  'x, x-y, z'
  7  'x+1/3, y+2/3, z+2/3'
  8  '-y+1/3, x-y+2/3, z+2/3'
  9  '-x+y+1/3, -x+2/3, z+2/3'
  10  '-y+1/3, -x+2/3, z+2/3'
  11  '-x+y+1/3, y+2/3, z+2/3'
  12  'x+1/3, x-y+2/3, z+2/3'
  13  'x+2/3, y+1/3, z+1/3'
  14  '-y+2/3, x-y+1/3, z+1/3'
  15  '-x+y+2/3, -x+1/3, z+1/3'
  16  '-y+2/3, -x+1/3, z+1/3'
  17  '-x+y+2/3, y+1/3, z+1/3'
  18  'x+2/3, x-y+1/3, z+1/3'
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
  Hg  Hg0  3  0.0  0.0  0.16  1.0
  Hg  Hg1  3  0.0  0.0  0.32  1.0
  Hg  Hg2  3  0.0  0.0  0.47  1.0
  Se  Se3  3  0.0  0.0  0.0  1.0

Hg Hg Hg Se
Hg3Se crystallizes in the trigonal R3m space group. There are three inequivalent Hg sites. In the first Hg site, Hg(1) is bonded in a 1-coordinate geometry to one Hg(2), three equivalent Hg(3), and one Se(1) atom. The Hg(1)-Hg(2) bond length is 2.01 Å. All Hg(1)-Hg(3) bond lengths are 3.02 Å. The Hg(1)-Se(1) bond length is 2.01 Å. In the second Hg site, Hg(2) is bonded to one Hg(1), one Hg(3), and three equivalent Se(1) atoms to form HgHg2Se3 trigonal bipyramids that share corners with six equivalent Hg(2)Hg2Se3 trigonal bipyramids,  a cornercorner with one Se(1)Hg4 trigonal pyramid, and corners with three equivalent Hg(3)Hg4 trigonal pyramids. The Hg(2)-Hg(3) bond length is 2.01 Å. All Hg(2)-Se(1) bond lengths are 3.02 Å. In the third Hg site, Hg(3) is bonded to one Hg(2) and three equivalent Hg(1) atoms to form distorted HgHg4 trigonal pyramids that share corners with three equivalent Hg(2)Hg2Se3 trigonal bipyramids, corners with six equivalent Hg(3)Hg4 trigonal pyramids, and edges with three equivalent Se(1)Hg4 trigonal pyramids. Se(1) is bonded to one Hg(1) and three equivalent Hg(2) atoms to form distorted SeHg4 trigonal pyramids that share  a cornercorner with one Hg(2)Hg2Se3 trigonal bipyramid, corners with six equivalent Se(1)Hg4 trigonal pyramids, and edges with three equivalent Hg(3)Hg4 trigonal pyramids.