The crystal structure described by Mg5Si9 crystallizes in the triclinic P1 space group. There are ten inequivalent Mg sites. In the first Mg site, Mg(1) is bonded in a 5-coordinate geometry to one Si(10), one Si(11), one Si(14), one Si(17), one Si(2), and one Si(8) atom. In the second Mg site, Mg(2) is bonded in a 5-coordinate geometry to one Si(10), one Si(17), one Si(3), one Si(5), and one Si(7) atom. In the third Mg site, Mg(3) is bonded to one Si(10), one Si(14), one Si(17), one Si(18), and one Si(3) atom to form distorted MgSi5 trigonal bipyramids that share  a cornercorner with one Si(5)Mg4Si2 octahedra, corners with two equivalent Mg(6)Si5 tetrahedra, and  an edgeedge with one Mg(6)Si5 tetrahedra. The corner-sharing octahedral tilt angles are 71°. In the fourth Mg site, Mg(4) is bonded in a 4-coordinate geometry to one Si(13), one Si(18), one Si(3), one Si(5), and one Si(9) atom. In the fifth Mg site, Mg(5) is bonded in a 6-coordinate geometry to one Si(10), one Si(14), one Si(17), one Si(18), one Si(3), one Si(7), and one Si(8) atom. In the sixth Mg site, Mg(6) is bonded to one Si(10), one Si(13), one Si(17), one Si(18), and one Si(3) atom to form distorted MgSi5 tetrahedra that share  a cornercorner with one Si(5)Mg4Si2 octahedra, corners with two equivalent Mg(3)Si5 trigonal bipyramids, and  an edgeedge with one Mg(3)Si5 trigonal bipyramid. The corner-sharing octahedral tilt angles are 81°. In the seventh Mg site, Mg(7) is bonded in a 8-coordinate geometry to one Si(1), one Si(12), one Si(13), one Si(15), one Si(16), one Si(5), one Si(7), and one Si(9) atom. In the eighth Mg site, Mg(8) is bonded in a 8-coordinate geometry to one Si(11), one Si(12), one Si(14), one Si(15), one Si(16), one Si(2), one Si(4), and one Si(8) atom. In the ninth Mg site, Mg(9) is bonded in a 6-coordinate geometry to one Si(13), one Si(15), one Si(3), one Si(5), one Si(7), and one Si(9) atom. In the tenth Mg site, Mg(10) is bonded in a 5-coordinate geometry to one Si(12), one Si(15), one Si(16), one Si(6), and one Si(8) atom. There are eighteen inequivalent Si sites. In the first Si site, Si(1) is bonded in a 5-coordinate geometry to one Mg(7), one Si(12), one Si(13), one Si(16), and one Si(5) atom. In the second Si site, Si(2) is bonded in a 6-coordinate geometry to one Mg(1), one Mg(8), one Si(11), one Si(14), one Si(15), and one Si(16) atom. In the third Si site, Si(3) is bonded in a 9-coordinate geometry to one Mg(2), one Mg(3), one Mg(4), one Mg(5), one Mg(6), one Mg(9), one Si(13), one Si(7), and one Si(9) atom. In the fourth Si site, Si(4) is bonded in a 5-coordinate geometry to one Mg(8), one Si(10), one Si(11), one Si(14), and one Si(17) atom. In the fifth Si site, Si(5) is bonded to one Mg(2), one Mg(4), one Mg(7), one Mg(9), one Si(1), and one Si(18) atom to form distorted SiMg4Si2 octahedra that share  a cornercorner with one Si(10)Mg5Si2 pentagonal bipyramid,  a cornercorner with one Mg(6)Si5 tetrahedra, and  a cornercorner with one Mg(3)Si5 trigonal bipyramid. In the sixth Si site, Si(6) is bonded in a 5-coordinate geometry to one Mg(10), one Si(10), one Si(11), one Si(15), and one Si(8) atom. In the seventh Si site, Si(7) is bonded in a 8-coordinate geometry to one Mg(2), one Mg(5), one Mg(7), one Mg(9), one Si(13), one Si(18), one Si(3), and one Si(9) atom. In the eighth Si site, Si(8) is bonded in a 7-coordinate geometry to one Mg(1), one Mg(10), one Mg(5), one Mg(8), one Si(14), one Si(17), and one Si(6) atom. In the ninth Si site, Si(9) is bonded in a 7-coordinate geometry to one Mg(4), one Mg(7), one Mg(9), one Si(12), one Si(16), one Si(3), and one Si(7) atom. In the tenth Si site, Si(10) is bonded to one Mg(1), one Mg(2), one Mg(3), one Mg(5), one Mg(6), one Si(4), and one Si(6) atom to form distorted corner-sharing SiMg5Si2 pentagonal bipyramids. The corner-sharing octahedral tilt angles are 50°. In the eleventh Si site, Si(11) is bonded in a 6-coordinate geometry to one Mg(1), one Mg(8), one Si(12), one Si(2), one Si(4), and one Si(6) atom. In the twelfth Si site, Si(12) is bonded in a 6-coordinate geometry to one Mg(10), one Mg(7), one Mg(8), one Si(1), one Si(11), and one Si(9) atom. In the thirteenth Si site, Si(13) is bonded in a 7-coordinate geometry to one Mg(4), one Mg(6), one Mg(7), one Mg(9), one Si(1), one Si(3), and one Si(7) atom. In the fourteenth Si site, Si(14) is bonded in a 7-coordinate geometry to one Mg(1), one Mg(3), one Mg(5), one Mg(8), one Si(2), one Si(4), and one Si(8) atom. In the fifteenth Si site, Si(15) is bonded in a 6-coordinate geometry to one Mg(10), one Mg(7), one Mg(8), one Mg(9), one Si(2), and one Si(6) atom. In the sixteenth Si site, Si(16) is bonded in a 6-coordinate geometry to one Mg(10), one Mg(7), one Mg(8), one Si(1), one Si(2), and one Si(9) atom. In the seventeenth Si site, Si(17) is bonded in a 7-coordinate geometry to one Mg(1), one Mg(2), one Mg(3), one Mg(5), one Mg(6), one Si(4), and one Si(8) atom. In the eighteenth Si site, Si(18) is bonded in a 6-coordinate geometry to one Mg(3), one Mg(4), one Mg(5), one Mg(6), one Si(5), and one Si(7) atom. is represented by the CIF card [CIF]
data_Mg5Si9
_symmetry_space_group_name_H-M   'P 1'
_cell_length_a   6.709
_cell_length_b   7.490
_cell_length_c   12.065
_cell_angle_alpha   84.002
_cell_angle_beta   82.325
_cell_angle_gamma   63.543
_symmetry_Int_Tables_number   1
_cell_volume   537.187
_cell_formula_units_Z   2
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  0.508  0.022  0.287  1.0
  Mg  Mg1  1  0.881  0.894  0.559  1.0
  Mg  Mg2  1  0.183  0.344  0.478  1.0
  Mg  Mg3  1  0.722  0.567  0.674  1.0
  Mg  Mg4  1  0.691  0.343  0.456  1.0
  Mg  Mg5  1  0.397  0.825  0.511  1.0
  Mg  Mg6  1  0.599  0.913  0.894  1.0
  Mg  Mg7  1  0.346  0.803  0.110  1.0
  Mg  Mg8  1  0.138  0.306  0.837  1.0
  Mg  Mg9  1  0.938  0.584  0.038  1.0
  Si  Si10  1  0.277  0.764  0.851  1.0
  Si  Si11  1  0.495  0.354  0.095  1.0
  Si  Si12  1  0.992  0.165  0.664  1.0
  Si  Si13  1  0.055  0.788  0.299  1.0
  Si  Si14  1  0.045  0.678  0.755  1.0
  Si  Si15  1  0.007  0.251  0.206  1.0
  Si  Si16  1  0.578  0.189  0.692  1.0
  Si  Si17  1  0.702  0.580  0.248  1.0
  Si  Si18  1  0.761  0.218  0.844  1.0
  Si  Si19  1  0.089  0.054  0.382  1.0
  Si  Si20  1  0.836  0.065  0.152  1.0
  Si  Si21  1  0.944  0.940  0.972  1.0
  Si  Si22  1  0.380  0.976  0.705  1.0
  Si  Si23  1  0.400  0.477  0.279  1.0
  Si  Si24  1  0.280  0.201  0.049  1.0
  Si  Si25  1  0.604  0.528  0.939  1.0
  Si  Si26  1  0.812  0.667  0.410  1.0
  Si  Si27  1  0.339  0.506  0.614  1.0
[/CIF]
.
The crystal structure described by Mg5Si9 crystallizes in the triclinic P1 space group. There are ten inequivalent Mg sites. In the first Mg site, Mg(1) is bonded in a 5-coordinate geometry to one Si(10), one Si(11), one Si(14), one Si(17), one Si(2), and one Si(8) atom. In the second Mg site, Mg(2) is bonded in a 5-coordinate geometry to one Si(10), one Si(17), one Si(3), one Si(5), and one Si(7) atom. In the third Mg site, Mg(3) is bonded to one Si(10), one Si(14), one Si(17), one Si(18), and one Si(3) atom to form distorted MgSi5 trigonal bipyramids that share  a cornercorner with one Si(5)Mg4Si2 octahedra, corners with two equivalent Mg(6)Si5 tetrahedra, and  an edgeedge with one Mg(6)Si5 tetrahedra. The corner-sharing octahedral tilt angles are 71°. In the fourth Mg site, Mg(4) is bonded in a 4-coordinate geometry to one Si(13), one Si(18), one Si(3), one Si(5), and one Si(9) atom. In the fifth Mg site, Mg(5) is bonded in a 6-coordinate geometry to one Si(10), one Si(14), one Si(17), one Si(18), one Si(3), one Si(7), and one Si(8) atom. In the sixth Mg site, Mg(6) is bonded to one Si(10), one Si(13), one Si(17), one Si(18), and one Si(3) atom to form distorted MgSi5 tetrahedra that share  a cornercorner with one Si(5)Mg4Si2 octahedra, corners with two equivalent Mg(3)Si5 trigonal bipyramids, and  an edgeedge with one Mg(3)Si5 trigonal bipyramid. The corner-sharing octahedral tilt angles are 81°. In the seventh Mg site, Mg(7) is bonded in a 8-coordinate geometry to one Si(1), one Si(12), one Si(13), one Si(15), one Si(16), one Si(5), one Si(7), and one Si(9) atom. In the eighth Mg site, Mg(8) is bonded in a 8-coordinate geometry to one Si(11), one Si(12), one Si(14), one Si(15), one Si(16), one Si(2), one Si(4), and one Si(8) atom. In the ninth Mg site, Mg(9) is bonded in a 6-coordinate geometry to one Si(13), one Si(15), one Si(3), one Si(5), one Si(7), and one Si(9) atom. In the tenth Mg site, Mg(10) is bonded in a 5-coordinate geometry to one Si(12), one Si(15), one Si(16), one Si(6), and one Si(8) atom. There are eighteen inequivalent Si sites. In the first Si site, Si(1) is bonded in a 5-coordinate geometry to one Mg(7), one Si(12), one Si(13), one Si(16), and one Si(5) atom. In the second Si site, Si(2) is bonded in a 6-coordinate geometry to one Mg(1), one Mg(8), one Si(11), one Si(14), one Si(15), and one Si(16) atom. In the third Si site, Si(3) is bonded in a 9-coordinate geometry to one Mg(2), one Mg(3), one Mg(4), one Mg(5), one Mg(6), one Mg(9), one Si(13), one Si(7), and one Si(9) atom. In the fourth Si site, Si(4) is bonded in a 5-coordinate geometry to one Mg(8), one Si(10), one Si(11), one Si(14), and one Si(17) atom. In the fifth Si site, Si(5) is bonded to one Mg(2), one Mg(4), one Mg(7), one Mg(9), one Si(1), and one Si(18) atom to form distorted SiMg4Si2 octahedra that share  a cornercorner with one Si(10)Mg5Si2 pentagonal bipyramid,  a cornercorner with one Mg(6)Si5 tetrahedra, and  a cornercorner with one Mg(3)Si5 trigonal bipyramid. In the sixth Si site, Si(6) is bonded in a 5-coordinate geometry to one Mg(10), one Si(10), one Si(11), one Si(15), and one Si(8) atom. In the seventh Si site, Si(7) is bonded in a 8-coordinate geometry to one Mg(2), one Mg(5), one Mg(7), one Mg(9), one Si(13), one Si(18), one Si(3), and one Si(9) atom. In the eighth Si site, Si(8) is bonded in a 7-coordinate geometry to one Mg(1), one Mg(10), one Mg(5), one Mg(8), one Si(14), one Si(17), and one Si(6) atom. In the ninth Si site, Si(9) is bonded in a 7-coordinate geometry to one Mg(4), one Mg(7), one Mg(9), one Si(12), one Si(16), one Si(3), and one Si(7) atom. In the tenth Si site, Si(10) is bonded to one Mg(1), one Mg(2), one Mg(3), one Mg(5), one Mg(6), one Si(4), and one Si(6) atom to form distorted corner-sharing SiMg5Si2 pentagonal bipyramids. The corner-sharing octahedral tilt angles are 50°. In the eleventh Si site, Si(11) is bonded in a 6-coordinate geometry to one Mg(1), one Mg(8), one Si(12), one Si(2), one Si(4), and one Si(6) atom. In the twelfth Si site, Si(12) is bonded in a 6-coordinate geometry to one Mg(10), one Mg(7), one Mg(8), one Si(1), one Si(11), and one Si(9) atom. In the thirteenth Si site, Si(13) is bonded in a 7-coordinate geometry to one Mg(4), one Mg(6), one Mg(7), one Mg(9), one Si(1), one Si(3), and one Si(7) atom. In the fourteenth Si site, Si(14) is bonded in a 7-coordinate geometry to one Mg(1), one Mg(3), one Mg(5), one Mg(8), one Si(2), one Si(4), and one Si(8) atom. In the fifteenth Si site, Si(15) is bonded in a 6-coordinate geometry to one Mg(10), one Mg(7), one Mg(8), one Mg(9), one Si(2), and one Si(6) atom. In the sixteenth Si site, Si(16) is bonded in a 6-coordinate geometry to one Mg(10), one Mg(7), one Mg(8), one Si(1), one Si(2), and one Si(9) atom. In the seventeenth Si site, Si(17) is bonded in a 7-coordinate geometry to one Mg(1), one Mg(2), one Mg(3), one Mg(5), one Mg(6), one Si(4), and one Si(8) atom. In the eighteenth Si site, Si(18) is bonded in a 6-coordinate geometry to one Mg(3), one Mg(4), one Mg(5), one Mg(6), one Si(5), and one Si(7) atom. is represented by the CIF card [CIF]
data_Mg5Si9
_symmetry_space_group_name_H-M   'P 1'
_cell_length_a   6.709
_cell_length_b   7.490
_cell_length_c   12.065
_cell_angle_alpha   84.002
_cell_angle_beta   82.325
_cell_angle_gamma   63.543
_symmetry_Int_Tables_number   1
_cell_volume   537.187
_cell_formula_units_Z   2
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  0.508  0.022  0.287  1.0
  Mg  Mg1  1  0.881  0.894  0.559  1.0
  Mg  Mg2  1  0.183  0.344  0.478  1.0
  Mg  Mg3  1  0.722  0.567  0.674  1.0
  Mg  Mg4  1  0.691  0.343  0.456  1.0
  Mg  Mg5  1  0.397  0.825  0.511  1.0
  Mg  Mg6  1  0.599  0.913  0.894  1.0
  Mg  Mg7  1  0.346  0.803  0.110  1.0
  Mg  Mg8  1  0.138  0.306  0.837  1.0
  Mg  Mg9  1  0.938  0.584  0.038  1.0
  Si  Si10  1  0.277  0.764  0.851  1.0
  Si  Si11  1  0.495  0.354  0.095  1.0
  Si  Si12  1  0.992  0.165  0.664  1.0
  Si  Si13  1  0.055  0.788  0.299  1.0
  Si  Si14  1  0.045  0.678  0.755  1.0
  Si  Si15  1  0.007  0.251  0.206  1.0
  Si  Si16  1  0.578  0.189  0.692  1.0
  Si  Si17  1  0.702  0.580  0.248  1.0
  Si  Si18  1  0.761  0.218  0.844  1.0
  Si  Si19  1  0.089  0.054  0.382  1.0
  Si  Si20  1  0.836  0.065  0.152  1.0
  Si  Si21  1  0.944  0.940  0.972  1.0
  Si  Si22  1  0.380  0.976  0.705  1.0
  Si  Si23  1  0.400  0.477  0.279  1.0
  Si  Si24  1  0.280  0.201  0.049  1.0
  Si  Si25  1  0.604  0.528  0.939  1.0
  Si  Si26  1  0.812  0.667  0.410  1.0
  Si  Si27  1  0.339  0.506  0.614  1.0
[/CIF]
.