The material described by MgSi2 crystallizes in the monoclinic C2/m space group. There are three inequivalent Mg sites. In the first Mg site, Mg(1) is bonded to four equivalent Mg(2), two equivalent Si(1), and six equivalent Si(2) atoms to form distorted MgMg4Si8 cuboctahedra that share corners with two equivalent Mg(3)Si12 cuboctahedra, corners with six equivalent Mg(2)Mg2Si10 cuboctahedra, edges with four equivalent Mg(1)Mg4Si8 cuboctahedra, faces with two equivalent Mg(1)Mg4Si8 cuboctahedra, and faces with six equivalent Mg(2)Mg2Si10 cuboctahedra. In the second Mg site, Mg(2) is bonded to two equivalent Mg(1), one Si(3), two equivalent Si(4), three equivalent Si(1), and four equivalent Si(2) atoms to form distorted MgMg2Si10 cuboctahedra that share corners with two equivalent Mg(2)Mg2Si10 cuboctahedra, corners with two equivalent Mg(3)Si12 cuboctahedra, corners with three equivalent Mg(1)Mg4Si8 cuboctahedra, edges with two equivalent Mg(3)Si12 cuboctahedra, edges with six equivalent Mg(2)Mg2Si10 cuboctahedra,  a faceface with one Mg(3)Si12 cuboctahedra, faces with three equivalent Mg(2)Mg2Si10 cuboctahedra, and faces with three equivalent Mg(1)Mg4Si8 cuboctahedra. In the third Mg site, Mg(3) is bonded to two equivalent Si(1), four equivalent Si(4), and six equivalent Si(3) atoms to form MgSi12 cuboctahedra that share corners with two equivalent Mg(1)Mg4Si8 cuboctahedra, corners with four equivalent Mg(2)Mg2Si10 cuboctahedra, edges with four equivalent Mg(2)Mg2Si10 cuboctahedra, edges with four equivalent Mg(3)Si12 cuboctahedra, faces with two equivalent Mg(2)Mg2Si10 cuboctahedra, and faces with two equivalent Mg(3)Si12 cuboctahedra. There are four inequivalent Si sites. In the first Si site, Si(1) is bonded in a 9-coordinate geometry to one Mg(1), one Mg(3), three equivalent Mg(2), two equivalent Si(3), and two equivalent Si(4) atoms. In the second Si site, Si(2) is bonded in a 9-coordinate geometry to three equivalent Mg(1), four equivalent Mg(2), and two equivalent Si(2) atoms. In the third Si site, Si(3) is bonded in a 4-coordinate geometry to one Mg(2), three equivalent Mg(3), and two equivalent Si(1) atoms. In the fourth Si site, Si(4) is bonded in a 4-coordinate geometry to two equivalent Mg(2), two equivalent Mg(3), and two equivalent Si(1) atoms. is represented by the Crystallographic Information File (CIF) [CIF]
data_MgSi2
_symmetry_space_group_name_H-M   'P 1'
_cell_length_a   5.001
_cell_length_b   5.001
_cell_length_c   11.513
_cell_angle_alpha   74.495
_cell_angle_beta   74.495
_cell_angle_gamma   45.841
_symmetry_Int_Tables_number   1
_cell_volume   197.723
_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
  Mg  Mg0  1  0.500  0.500  0.500  1.0
  Mg  Mg1  1  0.825  0.825  0.676  1.0
  Mg  Mg2  1  0.500  0.500  0.000  1.0
  Mg  Mg3  1  0.175  0.175  0.324  1.0
  Si  Si4  1  0.494  0.494  0.761  1.0
  Si  Si5  1  0.175  0.175  0.577  1.0
  Si  Si6  1  0.506  0.506  0.239  1.0
  Si  Si7  1  0.169  0.169  0.085  1.0
  Si  Si8  1  0.825  0.825  0.423  1.0
  Si  Si9  1  0.156  0.156  0.834  1.0
  Si  Si10  1  0.831  0.831  0.915  1.0
  Si  Si11  1  0.844  0.844  0.166  1.0
[/CIF]
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The material described by MgSi2 crystallizes in the monoclinic C2/m space group. There are three inequivalent Mg sites. In the first Mg site, Mg(1) is bonded to four equivalent Mg(2), two equivalent Si(1), and six equivalent Si(2) atoms to form distorted MgMg4Si8 cuboctahedra that share corners with two equivalent Mg(3)Si12 cuboctahedra, corners with six equivalent Mg(2)Mg2Si10 cuboctahedra, edges with four equivalent Mg(1)Mg4Si8 cuboctahedra, faces with two equivalent Mg(1)Mg4Si8 cuboctahedra, and faces with six equivalent Mg(2)Mg2Si10 cuboctahedra. In the second Mg site, Mg(2) is bonded to two equivalent Mg(1), one Si(3), two equivalent Si(4), three equivalent Si(1), and four equivalent Si(2) atoms to form distorted MgMg2Si10 cuboctahedra that share corners with two equivalent Mg(2)Mg2Si10 cuboctahedra, corners with two equivalent Mg(3)Si12 cuboctahedra, corners with three equivalent Mg(1)Mg4Si8 cuboctahedra, edges with two equivalent Mg(3)Si12 cuboctahedra, edges with six equivalent Mg(2)Mg2Si10 cuboctahedra,  a faceface with one Mg(3)Si12 cuboctahedra, faces with three equivalent Mg(2)Mg2Si10 cuboctahedra, and faces with three equivalent Mg(1)Mg4Si8 cuboctahedra. In the third Mg site, Mg(3) is bonded to two equivalent Si(1), four equivalent Si(4), and six equivalent Si(3) atoms to form MgSi12 cuboctahedra that share corners with two equivalent Mg(1)Mg4Si8 cuboctahedra, corners with four equivalent Mg(2)Mg2Si10 cuboctahedra, edges with four equivalent Mg(2)Mg2Si10 cuboctahedra, edges with four equivalent Mg(3)Si12 cuboctahedra, faces with two equivalent Mg(2)Mg2Si10 cuboctahedra, and faces with two equivalent Mg(3)Si12 cuboctahedra. There are four inequivalent Si sites. In the first Si site, Si(1) is bonded in a 9-coordinate geometry to one Mg(1), one Mg(3), three equivalent Mg(2), two equivalent Si(3), and two equivalent Si(4) atoms. In the second Si site, Si(2) is bonded in a 9-coordinate geometry to three equivalent Mg(1), four equivalent Mg(2), and two equivalent Si(2) atoms. In the third Si site, Si(3) is bonded in a 4-coordinate geometry to one Mg(2), three equivalent Mg(3), and two equivalent Si(1) atoms. In the fourth Si site, Si(4) is bonded in a 4-coordinate geometry to two equivalent Mg(2), two equivalent Mg(3), and two equivalent Si(1) atoms. is represented by the Crystallographic Information File (CIF) [CIF]
data_MgSi2
_symmetry_space_group_name_H-M   'P 1'
_cell_length_a   5.001
_cell_length_b   5.001
_cell_length_c   11.513
_cell_angle_alpha   74.495
_cell_angle_beta   74.495
_cell_angle_gamma   45.841
_symmetry_Int_Tables_number   1
_cell_volume   197.723
_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
  Mg  Mg0  1  0.500  0.500  0.500  1.0
  Mg  Mg1  1  0.825  0.825  0.676  1.0
  Mg  Mg2  1  0.500  0.500  0.000  1.0
  Mg  Mg3  1  0.175  0.175  0.324  1.0
  Si  Si4  1  0.494  0.494  0.761  1.0
  Si  Si5  1  0.175  0.175  0.577  1.0
  Si  Si6  1  0.506  0.506  0.239  1.0
  Si  Si7  1  0.169  0.169  0.085  1.0
  Si  Si8  1  0.825  0.825  0.423  1.0
  Si  Si9  1  0.156  0.156  0.834  1.0
  Si  Si10  1  0.831  0.831  0.915  1.0
  Si  Si11  1  0.844  0.844  0.166  1.0
[/CIF]
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