The structure described by MgTb2S4 crystallizes in the orthorhombic Pnma space group. Mg(1) is bonded to two equivalent S(4) and three equivalent S(1) atoms to form MgS5 square pyramids that share corners with two equivalent Tb(1)S6 octahedra, corners with two equivalent Tb(2)S6 octahedra,  an edgeedge with one Tb(1)S6 octahedra, edges with three equivalent Tb(2)S6 octahedra, and edges with four equivalent Mg(1)S5 square pyramids. The corner-sharing octahedral tilt angles range from 4-5°. There are two inequivalent Tb sites. In the first Tb site, Tb(1) is bonded to one S(3), two equivalent S(4), and three equivalent S(2) atoms to form TbS6 octahedra that share corners with three equivalent Tb(2)S6 octahedra, corners with two equivalent Mg(1)S5 square pyramids, edges with two equivalent Tb(2)S6 octahedra, edges with four equivalent Tb(1)S6 octahedra, and  an edgeedge with one Mg(1)S5 square pyramid. The corner-sharing octahedral tilt angles range from 54-59°. In the second Tb site, Tb(2) is bonded to one S(2), one S(4), two equivalent S(1), and two equivalent S(3) atoms to form TbS6 octahedra that share corners with three equivalent Tb(1)S6 octahedra, corners with two equivalent Mg(1)S5 square pyramids, edges with two equivalent Tb(1)S6 octahedra, edges with two equivalent Tb(2)S6 octahedra, and edges with three equivalent Mg(1)S5 square pyramids. The corner-sharing octahedral tilt angles range from 54-59°. There are four inequivalent S sites. In the first S site, S(1) is bonded to three equivalent Mg(1) and two equivalent Tb(2) atoms to form STb2Mg3 square pyramids that share corners with two equivalent S(4)Tb3Mg2 square pyramids, edges with three equivalent S(4)Tb3Mg2 square pyramids, and edges with four equivalent S(1)Tb2Mg3 square pyramids. In the second S site, S(2) is bonded in a rectangular see-saw-like geometry to one Tb(2) and three equivalent Tb(1) atoms. In the third S site, S(3) is bonded in a distorted T-shaped geometry to one Tb(1) and two equivalent Tb(2) atoms. In the fourth S site, S(4) is bonded to two equivalent Mg(1), one Tb(2), and two equivalent Tb(1) atoms to form STb3Mg2 square pyramids that share corners with two equivalent S(1)Tb2Mg3 square pyramids, edges with two equivalent S(4)Tb3Mg2 square pyramids, and edges with three equivalent S(1)Tb2Mg3 square pyramids. is represented by the CIF file [CIF]
data_Tb2MgS4
_symmetry_space_group_name_H-M   'P 1'
_cell_length_a   3.865
_cell_length_b   12.653
_cell_length_c   13.493
_cell_angle_alpha   90.000
_cell_angle_beta   90.000
_cell_angle_gamma   90.000
_symmetry_Int_Tables_number   1
_cell_volume   659.801
_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  0.250  0.118  0.076  1.0
  Tb  Tb1  1  0.750  0.882  0.924  1.0
  Tb  Tb2  1  0.750  0.382  0.576  1.0
  Tb  Tb3  1  0.250  0.618  0.424  1.0
  Tb  Tb4  1  0.250  0.139  0.706  1.0
  Tb  Tb5  1  0.750  0.861  0.294  1.0
  Tb  Tb6  1  0.750  0.361  0.206  1.0
  Tb  Tb7  1  0.250  0.639  0.794  1.0
  Mg  Mg8  1  0.750  0.111  0.453  1.0
  Mg  Mg9  1  0.250  0.889  0.547  1.0
  Mg  Mg10  1  0.250  0.389  0.953  1.0
  Mg  Mg11  1  0.750  0.611  0.047  1.0
  S  S12  1  0.750  0.008  0.615  1.0
  S  S13  1  0.250  0.992  0.385  1.0
  S  S14  1  0.250  0.492  0.115  1.0
  S  S15  1  0.750  0.508  0.885  1.0
  S  S16  1  0.250  0.027  0.883  1.0
  S  S17  1  0.750  0.973  0.117  1.0
  S  S18  1  0.750  0.473  0.383  1.0
  S  S19  1  0.250  0.527  0.617  1.0
  S  S20  1  0.250  0.225  0.249  1.0
  S  S21  1  0.750  0.775  0.751  1.0
  S  S22  1  0.750  0.275  0.749  1.0
  S  S23  1  0.250  0.725  0.251  1.0
  S  S24  1  0.750  0.262  0.016  1.0
  S  S25  1  0.250  0.738  0.984  1.0
  S  S26  1  0.250  0.238  0.516  1.0
  S  S27  1  0.750  0.762  0.484  1.0
[/CIF]
.
The structure described by MgTb2S4 crystallizes in the orthorhombic Pnma space group. Mg(1) is bonded to two equivalent S(4) and three equivalent S(1) atoms to form MgS5 square pyramids that share corners with two equivalent Tb(1)S6 octahedra, corners with two equivalent Tb(2)S6 octahedra,  an edgeedge with one Tb(1)S6 octahedra, edges with three equivalent Tb(2)S6 octahedra, and edges with four equivalent Mg(1)S5 square pyramids. The corner-sharing octahedral tilt angles range from 4-5°. There are two inequivalent Tb sites. In the first Tb site, Tb(1) is bonded to one S(3), two equivalent S(4), and three equivalent S(2) atoms to form TbS6 octahedra that share corners with three equivalent Tb(2)S6 octahedra, corners with two equivalent Mg(1)S5 square pyramids, edges with two equivalent Tb(2)S6 octahedra, edges with four equivalent Tb(1)S6 octahedra, and  an edgeedge with one Mg(1)S5 square pyramid. The corner-sharing octahedral tilt angles range from 54-59°. In the second Tb site, Tb(2) is bonded to one S(2), one S(4), two equivalent S(1), and two equivalent S(3) atoms to form TbS6 octahedra that share corners with three equivalent Tb(1)S6 octahedra, corners with two equivalent Mg(1)S5 square pyramids, edges with two equivalent Tb(1)S6 octahedra, edges with two equivalent Tb(2)S6 octahedra, and edges with three equivalent Mg(1)S5 square pyramids. The corner-sharing octahedral tilt angles range from 54-59°. There are four inequivalent S sites. In the first S site, S(1) is bonded to three equivalent Mg(1) and two equivalent Tb(2) atoms to form STb2Mg3 square pyramids that share corners with two equivalent S(4)Tb3Mg2 square pyramids, edges with three equivalent S(4)Tb3Mg2 square pyramids, and edges with four equivalent S(1)Tb2Mg3 square pyramids. In the second S site, S(2) is bonded in a rectangular see-saw-like geometry to one Tb(2) and three equivalent Tb(1) atoms. In the third S site, S(3) is bonded in a distorted T-shaped geometry to one Tb(1) and two equivalent Tb(2) atoms. In the fourth S site, S(4) is bonded to two equivalent Mg(1), one Tb(2), and two equivalent Tb(1) atoms to form STb3Mg2 square pyramids that share corners with two equivalent S(1)Tb2Mg3 square pyramids, edges with two equivalent S(4)Tb3Mg2 square pyramids, and edges with three equivalent S(1)Tb2Mg3 square pyramids. is represented by the CIF file [CIF]
data_Tb2MgS4
_symmetry_space_group_name_H-M   'P 1'
_cell_length_a   3.865
_cell_length_b   12.653
_cell_length_c   13.493
_cell_angle_alpha   90.000
_cell_angle_beta   90.000
_cell_angle_gamma   90.000
_symmetry_Int_Tables_number   1
_cell_volume   659.801
_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  0.250  0.118  0.076  1.0
  Tb  Tb1  1  0.750  0.882  0.924  1.0
  Tb  Tb2  1  0.750  0.382  0.576  1.0
  Tb  Tb3  1  0.250  0.618  0.424  1.0
  Tb  Tb4  1  0.250  0.139  0.706  1.0
  Tb  Tb5  1  0.750  0.861  0.294  1.0
  Tb  Tb6  1  0.750  0.361  0.206  1.0
  Tb  Tb7  1  0.250  0.639  0.794  1.0
  Mg  Mg8  1  0.750  0.111  0.453  1.0
  Mg  Mg9  1  0.250  0.889  0.547  1.0
  Mg  Mg10  1  0.250  0.389  0.953  1.0
  Mg  Mg11  1  0.750  0.611  0.047  1.0
  S  S12  1  0.750  0.008  0.615  1.0
  S  S13  1  0.250  0.992  0.385  1.0
  S  S14  1  0.250  0.492  0.115  1.0
  S  S15  1  0.750  0.508  0.885  1.0
  S  S16  1  0.250  0.027  0.883  1.0
  S  S17  1  0.750  0.973  0.117  1.0
  S  S18  1  0.750  0.473  0.383  1.0
  S  S19  1  0.250  0.527  0.617  1.0
  S  S20  1  0.250  0.225  0.249  1.0
  S  S21  1  0.750  0.775  0.751  1.0
  S  S22  1  0.750  0.275  0.749  1.0
  S  S23  1  0.250  0.725  0.251  1.0
  S  S24  1  0.750  0.262  0.016  1.0
  S  S25  1  0.250  0.738  0.984  1.0
  S  S26  1  0.250  0.238  0.516  1.0
  S  S27  1  0.750  0.762  0.484  1.0
[/CIF]
.