The material described by MgTb2Se4 is Spinel-like structured and crystallizes in the orthorhombic Pnma space group. Mg(1) is bonded to one Se(2), one Se(3), and two equivalent Se(1) atoms to form MgSe4 tetrahedra that share corners with two equivalent Tb(1)Se6 octahedra, corners with four equivalent Tb(2)Se6 octahedra,  an edgeedge with one Tb(2)Se6 octahedra, and edges with two equivalent Tb(1)Se6 octahedra. The corner-sharing octahedral tilt angles range from 58-66°. There are two inequivalent Tb sites. In the first Tb site, Tb(1) is bonded to two equivalent Se(1), two equivalent Se(2), and two equivalent Se(3) atoms to form TbSe6 octahedra that share corners with four equivalent Tb(2)Se6 octahedra, corners with two equivalent Mg(1)Se4 tetrahedra, edges with two equivalent Tb(1)Se6 octahedra, edges with two equivalent Tb(2)Se6 octahedra, and edges with two equivalent Mg(1)Se4 tetrahedra. The corner-sharing octahedral tilt angles range from 50-54°. In the second Tb site, Tb(2) is bonded to one Se(2), one Se(3), and four equivalent Se(1) atoms to form TbSe6 octahedra that share corners with four equivalent Tb(1)Se6 octahedra, corners with four equivalent Tb(2)Se6 octahedra, corners with four equivalent Mg(1)Se4 tetrahedra, edges with two equivalent Tb(1)Se6 octahedra, and  an edgeedge with one Mg(1)Se4 tetrahedra. The corner-sharing octahedral tilt angles range from 46-54°. There are three inequivalent Se sites. In the first Se site, Se(1) is bonded in a rectangular see-saw-like geometry to one Mg(1), one Tb(1), and two equivalent Tb(2) atoms. In the second Se site, Se(2) is bonded in a rectangular see-saw-like geometry to one Mg(1), one Tb(2), and two equivalent Tb(1) atoms. In the third Se site, Se(3) is bonded to one Mg(1), one Tb(2), and two equivalent Tb(1) atoms to form distorted corner-sharing SeTb3Mg trigonal pyramids. is represented by the Crystallographic Information File (CIF) [CIF]
data_Tb2MgSe4
_symmetry_space_group_name_H-M   'P 1'
_cell_length_a   14.120
_cell_length_b   8.218
_cell_length_c   6.819
_cell_angle_alpha   90.000
_cell_angle_beta   90.000
_cell_angle_gamma   90.000
_symmetry_Int_Tables_number   1
_cell_volume   791.257
_cell_formula_units_Z   4
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
  Tb  Tb0  1  0.000  1.000  0.500  1.0
  Tb  Tb1  1  0.500  1.000  1.000  1.0
  Tb  Tb2  1  0.500  0.500  1.000  1.0
  Tb  Tb3  1  0.000  0.500  0.500  1.0
  Tb  Tb4  1  0.234  0.750  0.995  1.0
  Tb  Tb5  1  0.766  0.250  0.005  1.0
  Tb  Tb6  1  0.266  0.250  0.495  1.0
  Tb  Tb7  1  0.734  0.750  0.505  1.0
  Mg  Mg8  1  0.095  0.250  0.914  1.0
  Mg  Mg9  1  0.906  0.750  0.086  1.0
  Mg  Mg10  1  0.405  0.750  0.414  1.0
  Mg  Mg11  1  0.594  0.250  0.586  1.0
  Se  Se12  1  0.170  0.006  0.735  1.0
  Se  Se13  1  0.830  0.994  0.265  1.0
  Se  Se14  1  0.330  0.994  0.235  1.0
  Se  Se15  1  0.670  0.006  0.765  1.0
  Se  Se16  1  0.670  0.494  0.765  1.0
  Se  Se17  1  0.330  0.506  0.235  1.0
  Se  Se18  1  0.830  0.506  0.265  1.0
  Se  Se19  1  0.170  0.494  0.735  1.0
  Se  Se20  1  0.072  0.750  0.246  1.0
  Se  Se21  1  0.928  0.250  0.754  1.0
  Se  Se22  1  0.428  0.250  0.746  1.0
  Se  Se23  1  0.572  0.750  0.254  1.0
  Se  Se24  1  0.090  0.250  0.283  1.0
  Se  Se25  1  0.910  0.750  0.717  1.0
  Se  Se26  1  0.410  0.750  0.783  1.0
  Se  Se27  1  0.590  0.250  0.217  1.0
[/CIF]
.
The material described by MgTb2Se4 is Spinel-like structured and crystallizes in the orthorhombic Pnma space group. Mg(1) is bonded to one Se(2), one Se(3), and two equivalent Se(1) atoms to form MgSe4 tetrahedra that share corners with two equivalent Tb(1)Se6 octahedra, corners with four equivalent Tb(2)Se6 octahedra,  an edgeedge with one Tb(2)Se6 octahedra, and edges with two equivalent Tb(1)Se6 octahedra. The corner-sharing octahedral tilt angles range from 58-66°. There are two inequivalent Tb sites. In the first Tb site, Tb(1) is bonded to two equivalent Se(1), two equivalent Se(2), and two equivalent Se(3) atoms to form TbSe6 octahedra that share corners with four equivalent Tb(2)Se6 octahedra, corners with two equivalent Mg(1)Se4 tetrahedra, edges with two equivalent Tb(1)Se6 octahedra, edges with two equivalent Tb(2)Se6 octahedra, and edges with two equivalent Mg(1)Se4 tetrahedra. The corner-sharing octahedral tilt angles range from 50-54°. In the second Tb site, Tb(2) is bonded to one Se(2), one Se(3), and four equivalent Se(1) atoms to form TbSe6 octahedra that share corners with four equivalent Tb(1)Se6 octahedra, corners with four equivalent Tb(2)Se6 octahedra, corners with four equivalent Mg(1)Se4 tetrahedra, edges with two equivalent Tb(1)Se6 octahedra, and  an edgeedge with one Mg(1)Se4 tetrahedra. The corner-sharing octahedral tilt angles range from 46-54°. There are three inequivalent Se sites. In the first Se site, Se(1) is bonded in a rectangular see-saw-like geometry to one Mg(1), one Tb(1), and two equivalent Tb(2) atoms. In the second Se site, Se(2) is bonded in a rectangular see-saw-like geometry to one Mg(1), one Tb(2), and two equivalent Tb(1) atoms. In the third Se site, Se(3) is bonded to one Mg(1), one Tb(2), and two equivalent Tb(1) atoms to form distorted corner-sharing SeTb3Mg trigonal pyramids. is represented by the Crystallographic Information File (CIF) [CIF]
data_Tb2MgSe4
_symmetry_space_group_name_H-M   'P 1'
_cell_length_a   14.120
_cell_length_b   8.218
_cell_length_c   6.819
_cell_angle_alpha   90.000
_cell_angle_beta   90.000
_cell_angle_gamma   90.000
_symmetry_Int_Tables_number   1
_cell_volume   791.257
_cell_formula_units_Z   4
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
  Tb  Tb0  1  0.000  1.000  0.500  1.0
  Tb  Tb1  1  0.500  1.000  1.000  1.0
  Tb  Tb2  1  0.500  0.500  1.000  1.0
  Tb  Tb3  1  0.000  0.500  0.500  1.0
  Tb  Tb4  1  0.234  0.750  0.995  1.0
  Tb  Tb5  1  0.766  0.250  0.005  1.0
  Tb  Tb6  1  0.266  0.250  0.495  1.0
  Tb  Tb7  1  0.734  0.750  0.505  1.0
  Mg  Mg8  1  0.095  0.250  0.914  1.0
  Mg  Mg9  1  0.906  0.750  0.086  1.0
  Mg  Mg10  1  0.405  0.750  0.414  1.0
  Mg  Mg11  1  0.594  0.250  0.586  1.0
  Se  Se12  1  0.170  0.006  0.735  1.0
  Se  Se13  1  0.830  0.994  0.265  1.0
  Se  Se14  1  0.330  0.994  0.235  1.0
  Se  Se15  1  0.670  0.006  0.765  1.0
  Se  Se16  1  0.670  0.494  0.765  1.0
  Se  Se17  1  0.330  0.506  0.235  1.0
  Se  Se18  1  0.830  0.506  0.265  1.0
  Se  Se19  1  0.170  0.494  0.735  1.0
  Se  Se20  1  0.072  0.750  0.246  1.0
  Se  Se21  1  0.928  0.250  0.754  1.0
  Se  Se22  1  0.428  0.250  0.746  1.0
  Se  Se23  1  0.572  0.750  0.254  1.0
  Se  Se24  1  0.090  0.250  0.283  1.0
  Se  Se25  1  0.910  0.750  0.717  1.0
  Se  Se26  1  0.410  0.750  0.783  1.0
  Se  Se27  1  0.590  0.250  0.217  1.0
[/CIF]
.