The compound described by Er2Se3 crystallizes in the tetragonal I-42d space group. There are three inequivalent Er sites. In the first Er site, Er(1) is bonded in a 8-coordinate geometry to two equivalent Se(1), three equivalent Se(2), and three equivalent Se(3) atoms. In the second Er site, Er(2) is bonded in a 8-coordinate geometry to two equivalent Se(1), two equivalent Se(3), and four equivalent Se(2) atoms. In the third Er site, Er(3) is bonded in a 8-coordinate geometry to two equivalent Se(2), two equivalent Se(3), and four equivalent Se(1) atoms. There are three inequivalent Se sites. In the first Se site, Se(1) is bonded to one Er(2), two equivalent Er(1), and two equivalent Er(3) atoms to form distorted SeEr5 trigonal bipyramids that share corners with five equivalent Se(2)Er6 octahedra, corners with four equivalent Se(3)Er5 square pyramids, corners with four equivalent Se(1)Er5 trigonal bipyramids, edges with three equivalent Se(2)Er6 octahedra, edges with four equivalent Se(3)Er5 square pyramids,  an edgeedge with one Se(1)Er5 trigonal bipyramid,  a faceface with one Se(2)Er6 octahedra, and  a faceface with one Se(1)Er5 trigonal bipyramid. The corner-sharing octahedral tilt angles range from 24-50°. In the second Se site, Se(2) is bonded to one Er(3), two equivalent Er(2), and three equivalent Er(1) atoms to form distorted SeEr6 octahedra that share corners with three equivalent Se(2)Er6 octahedra, corners with seven equivalent Se(3)Er5 square pyramids, corners with five equivalent Se(1)Er5 trigonal bipyramids, edges with two equivalent Se(2)Er6 octahedra,  an edgeedge with one Se(3)Er5 square pyramid, edges with three equivalent Se(1)Er5 trigonal bipyramids, faces with two equivalent Se(2)Er6 octahedra, faces with two equivalent Se(3)Er5 square pyramids, and  a faceface with one Se(1)Er5 trigonal bipyramid. The corner-sharing octahedral tilt angles range from 19-49°. In the third Se site, Se(3) is bonded to one Er(2), one Er(3), and three equivalent Er(1) atoms to form distorted SeEr5 square pyramids that share corners with seven equivalent Se(2)Er6 octahedra, corners with five equivalent Se(3)Er5 square pyramids, corners with four equivalent Se(1)Er5 trigonal bipyramids,  an edgeedge with one Se(2)Er6 octahedra, edges with four equivalent Se(1)Er5 trigonal bipyramids, faces with two equivalent Se(2)Er6 octahedra, and  a faceface with one Se(3)Er5 square pyramid. The corner-sharing octahedral tilt angles range from 16-53°. is represented by the CIF file [CIF]
data_Er2Se3
_symmetry_space_group_name_H-M   'P 1'
_cell_length_a   14.317
_cell_length_b   14.317
_cell_length_c   14.317
_cell_angle_alpha   144.892
_cell_angle_beta   144.892
_cell_angle_gamma   50.495
_symmetry_Int_Tables_number   1
_cell_volume   965.858
_cell_formula_units_Z   8
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
  Er  Er0  1  0.577  0.471  0.626  1.0
  Er  Er1  1  0.167  0.167  0.000  1.0
  Er  Er2  1  0.875  0.508  0.133  1.0
  Er  Er3  1  0.173  0.299  0.394  1.0
  Er  Er4  1  0.583  0.083  0.500  1.0
  Er  Er5  1  0.905  0.779  0.606  1.0
  Er  Er6  1  0.375  0.742  0.867  1.0
  Er  Er7  1  0.221  0.827  0.126  1.0
  Er  Er8  1  0.492  0.625  0.367  1.0
  Er  Er9  1  0.917  0.417  0.500  1.0
  Er  Er10  1  0.258  0.125  0.633  1.0
  Er  Er11  1  0.701  0.095  0.874  1.0
  Er  Er12  1  0.833  0.833  0.000  1.0
  Er  Er13  1  0.529  0.155  0.106  1.0
  Er  Er14  1  0.845  0.951  0.374  1.0
  Er  Er15  1  0.049  0.423  0.894  1.0
  Se  Se16  1  0.075  0.828  0.614  1.0
  Se  Se17  1  0.865  0.250  0.745  1.0
  Se  Se18  1  0.505  0.119  0.255  1.0
  Se  Se19  1  0.885  0.131  0.885  1.0
  Se  Se20  1  0.245  0.000  0.115  1.0
  Se  Se21  1  0.214  0.461  0.386  1.0
  Se  Se22  1  0.536  0.422  0.747  1.0
  Se  Se23  1  0.404  0.160  0.598  1.0
  Se  Se24  1  0.195  0.596  0.756  1.0
  Se  Se25  1  0.675  0.789  0.253  1.0
  Se  Se26  1  0.840  0.439  0.244  1.0
  Se  Se27  1  0.211  0.464  0.886  1.0
  Se  Se28  1  0.578  0.325  0.114  1.0
  Se  Se29  1  0.561  0.805  0.402  1.0
  Se  Se30  1  0.869  0.755  0.755  1.0
  Se  Se31  1  0.750  0.495  0.615  1.0
  Se  Se32  1  0.539  0.925  0.753  1.0
  Se  Se33  1  1.000  0.115  0.245  1.0
  Se  Se34  1  0.555  0.811  0.902  1.0
  Se  Se35  1  0.172  0.786  0.247  1.0
  Se  Se36  1  0.910  0.654  0.098  1.0
  Se  Se37  1  0.881  0.135  0.385  1.0
  Se  Se38  1  0.189  0.090  0.744  1.0
  Se  Se39  1  0.346  0.445  0.256  1.0
[/CIF]
.
The compound described by Er2Se3 crystallizes in the tetragonal I-42d space group. There are three inequivalent Er sites. In the first Er site, Er(1) is bonded in a 8-coordinate geometry to two equivalent Se(1), three equivalent Se(2), and three equivalent Se(3) atoms. In the second Er site, Er(2) is bonded in a 8-coordinate geometry to two equivalent Se(1), two equivalent Se(3), and four equivalent Se(2) atoms. In the third Er site, Er(3) is bonded in a 8-coordinate geometry to two equivalent Se(2), two equivalent Se(3), and four equivalent Se(1) atoms. There are three inequivalent Se sites. In the first Se site, Se(1) is bonded to one Er(2), two equivalent Er(1), and two equivalent Er(3) atoms to form distorted SeEr5 trigonal bipyramids that share corners with five equivalent Se(2)Er6 octahedra, corners with four equivalent Se(3)Er5 square pyramids, corners with four equivalent Se(1)Er5 trigonal bipyramids, edges with three equivalent Se(2)Er6 octahedra, edges with four equivalent Se(3)Er5 square pyramids,  an edgeedge with one Se(1)Er5 trigonal bipyramid,  a faceface with one Se(2)Er6 octahedra, and  a faceface with one Se(1)Er5 trigonal bipyramid. The corner-sharing octahedral tilt angles range from 24-50°. In the second Se site, Se(2) is bonded to one Er(3), two equivalent Er(2), and three equivalent Er(1) atoms to form distorted SeEr6 octahedra that share corners with three equivalent Se(2)Er6 octahedra, corners with seven equivalent Se(3)Er5 square pyramids, corners with five equivalent Se(1)Er5 trigonal bipyramids, edges with two equivalent Se(2)Er6 octahedra,  an edgeedge with one Se(3)Er5 square pyramid, edges with three equivalent Se(1)Er5 trigonal bipyramids, faces with two equivalent Se(2)Er6 octahedra, faces with two equivalent Se(3)Er5 square pyramids, and  a faceface with one Se(1)Er5 trigonal bipyramid. The corner-sharing octahedral tilt angles range from 19-49°. In the third Se site, Se(3) is bonded to one Er(2), one Er(3), and three equivalent Er(1) atoms to form distorted SeEr5 square pyramids that share corners with seven equivalent Se(2)Er6 octahedra, corners with five equivalent Se(3)Er5 square pyramids, corners with four equivalent Se(1)Er5 trigonal bipyramids,  an edgeedge with one Se(2)Er6 octahedra, edges with four equivalent Se(1)Er5 trigonal bipyramids, faces with two equivalent Se(2)Er6 octahedra, and  a faceface with one Se(3)Er5 square pyramid. The corner-sharing octahedral tilt angles range from 16-53°. is represented by the CIF file [CIF]
data_Er2Se3
_symmetry_space_group_name_H-M   'P 1'
_cell_length_a   14.317
_cell_length_b   14.317
_cell_length_c   14.317
_cell_angle_alpha   144.892
_cell_angle_beta   144.892
_cell_angle_gamma   50.495
_symmetry_Int_Tables_number   1
_cell_volume   965.858
_cell_formula_units_Z   8
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
  Er  Er0  1  0.577  0.471  0.626  1.0
  Er  Er1  1  0.167  0.167  0.000  1.0
  Er  Er2  1  0.875  0.508  0.133  1.0
  Er  Er3  1  0.173  0.299  0.394  1.0
  Er  Er4  1  0.583  0.083  0.500  1.0
  Er  Er5  1  0.905  0.779  0.606  1.0
  Er  Er6  1  0.375  0.742  0.867  1.0
  Er  Er7  1  0.221  0.827  0.126  1.0
  Er  Er8  1  0.492  0.625  0.367  1.0
  Er  Er9  1  0.917  0.417  0.500  1.0
  Er  Er10  1  0.258  0.125  0.633  1.0
  Er  Er11  1  0.701  0.095  0.874  1.0
  Er  Er12  1  0.833  0.833  0.000  1.0
  Er  Er13  1  0.529  0.155  0.106  1.0
  Er  Er14  1  0.845  0.951  0.374  1.0
  Er  Er15  1  0.049  0.423  0.894  1.0
  Se  Se16  1  0.075  0.828  0.614  1.0
  Se  Se17  1  0.865  0.250  0.745  1.0
  Se  Se18  1  0.505  0.119  0.255  1.0
  Se  Se19  1  0.885  0.131  0.885  1.0
  Se  Se20  1  0.245  0.000  0.115  1.0
  Se  Se21  1  0.214  0.461  0.386  1.0
  Se  Se22  1  0.536  0.422  0.747  1.0
  Se  Se23  1  0.404  0.160  0.598  1.0
  Se  Se24  1  0.195  0.596  0.756  1.0
  Se  Se25  1  0.675  0.789  0.253  1.0
  Se  Se26  1  0.840  0.439  0.244  1.0
  Se  Se27  1  0.211  0.464  0.886  1.0
  Se  Se28  1  0.578  0.325  0.114  1.0
  Se  Se29  1  0.561  0.805  0.402  1.0
  Se  Se30  1  0.869  0.755  0.755  1.0
  Se  Se31  1  0.750  0.495  0.615  1.0
  Se  Se32  1  0.539  0.925  0.753  1.0
  Se  Se33  1  1.000  0.115  0.245  1.0
  Se  Se34  1  0.555  0.811  0.902  1.0
  Se  Se35  1  0.172  0.786  0.247  1.0
  Se  Se36  1  0.910  0.654  0.098  1.0
  Se  Se37  1  0.881  0.135  0.385  1.0
  Se  Se38  1  0.189  0.090  0.744  1.0
  Se  Se39  1  0.346  0.445  0.256  1.0
[/CIF]
.