The crystal structure described by Na8Si7O18 crystallizes in the trigonal R-3m space group. There are three inequivalent Na sites. In the first Na site, Na(1) is bonded to three equivalent O(2) and three equivalent O(3) atoms to form distorted NaO6 octahedra that share corners with six equivalent Si(2)O4 tetrahedra, faces with three equivalent Na(3)O8 hexagonal bipyramids, and  a faceface with one Si(1)O6 octahedra. In the second Na site, Na(2) is bonded in a body-centered cubic geometry to four equivalent O(1) and four equivalent O(3) atoms. In the third Na site, Na(3) is bonded to two equivalent O(1), two equivalent O(3), and four equivalent O(2) atoms to form distorted NaO8 hexagonal bipyramids that share corners with four equivalent Na(3)O8 hexagonal bipyramids, corners with two equivalent Si(2)O4 tetrahedra, edges with two equivalent Si(1)O6 octahedra, edges with four equivalent Si(2)O4 tetrahedra, and faces with two equivalent Na(1)O6 octahedra. There are two inequivalent Si sites. In the first Si site, Si(1) is bonded to six equivalent O(2) atoms to form SiO6 octahedra that share corners with six equivalent Si(2)O4 tetrahedra, edges with six equivalent Na(3)O8 hexagonal bipyramids, and faces with two equivalent Na(1)O6 octahedra. In the second Si site, Si(2) is bonded to one O(2), one O(3), and two equivalent O(1) atoms to form SiO4 tetrahedra that share  a cornercorner with one Na(3)O8 hexagonal bipyramid,  a cornercorner with one Si(1)O6 octahedra, corners with two equivalent Na(1)O6 octahedra, corners with two equivalent Si(2)O4 tetrahedra, and edges with two equivalent Na(3)O8 hexagonal bipyramids. The corner-sharing octahedral tilt angles range from 30-58°. There are three inequivalent O sites. In the first O site, O(1) is bonded to one Na(3), two equivalent Na(2), and two equivalent Si(2) atoms to form a mixture of distorted edge and corner-sharing ONa3Si2 trigonal bipyramids. In the second O site, O(2) is bonded in a 5-coordinate geometry to one Na(1), two equivalent Na(3), one Si(1), and one Si(2) atom. In the third O site, O(3) is bonded in a 5-coordinate geometry to one Na(1), one Na(3), two equivalent Na(2), and one Si(2) atom. is represented by the CIF card [CIF]
data_Na8Si7O18
_symmetry_space_group_name_H-M   'P 1'
_cell_length_a   7.259
_cell_length_b   7.259
_cell_length_c   7.259
_cell_angle_alpha   87.186
_cell_angle_beta   87.186
_cell_angle_gamma   87.186
_symmetry_Int_Tables_number   1
_cell_volume   381.161
_cell_formula_units_Z   1
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
  Na  Na0  1  0.236  0.236  0.236  1.0
  Na  Na1  1  0.764  0.764  0.764  1.0
  Na  Na2  1  0.500  0.500  0.000  1.0
  Na  Na3  1  0.500  0.000  0.500  1.0
  Na  Na4  1  0.000  0.500  0.500  1.0
  Na  Na5  1  0.000  0.000  0.500  1.0
  Na  Na6  1  0.000  0.500  0.000  1.0
  Na  Na7  1  0.500  0.000  0.000  1.0
  Si  Si8  1  0.000  0.000  0.000  1.0
  Si  Si9  1  0.272  0.739  0.272  1.0
  Si  Si10  1  0.739  0.272  0.272  1.0
  Si  Si11  1  0.272  0.272  0.739  1.0
  Si  Si12  1  0.728  0.261  0.728  1.0
  Si  Si13  1  0.261  0.728  0.728  1.0
  Si  Si14  1  0.728  0.728  0.261  1.0
  O  O15  1  0.766  0.500  0.234  1.0
  O  O16  1  0.234  0.766  0.500  1.0
  O  O17  1  0.500  0.234  0.766  1.0
  O  O18  1  0.234  0.500  0.766  1.0
  O  O19  1  0.766  0.234  0.500  1.0
  O  O20  1  0.500  0.766  0.234  1.0
  O  O21  1  0.831  0.831  0.081  1.0
  O  O22  1  0.081  0.831  0.831  1.0
  O  O23  1  0.831  0.081  0.831  1.0
  O  O24  1  0.169  0.169  0.919  1.0
  O  O25  1  0.919  0.169  0.169  1.0
  O  O26  1  0.169  0.919  0.169  1.0
  O  O27  1  0.787  0.787  0.456  1.0
  O  O28  1  0.456  0.787  0.787  1.0
  O  O29  1  0.787  0.456  0.787  1.0
  O  O30  1  0.213  0.544  0.213  1.0
  O  O31  1  0.544  0.213  0.213  1.0
  O  O32  1  0.213  0.213  0.544  1.0
[/CIF]
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The crystal structure described by Na8Si7O18 crystallizes in the trigonal R-3m space group. There are three inequivalent Na sites. In the first Na site, Na(1) is bonded to three equivalent O(2) and three equivalent O(3) atoms to form distorted NaO6 octahedra that share corners with six equivalent Si(2)O4 tetrahedra, faces with three equivalent Na(3)O8 hexagonal bipyramids, and  a faceface with one Si(1)O6 octahedra. In the second Na site, Na(2) is bonded in a body-centered cubic geometry to four equivalent O(1) and four equivalent O(3) atoms. In the third Na site, Na(3) is bonded to two equivalent O(1), two equivalent O(3), and four equivalent O(2) atoms to form distorted NaO8 hexagonal bipyramids that share corners with four equivalent Na(3)O8 hexagonal bipyramids, corners with two equivalent Si(2)O4 tetrahedra, edges with two equivalent Si(1)O6 octahedra, edges with four equivalent Si(2)O4 tetrahedra, and faces with two equivalent Na(1)O6 octahedra. There are two inequivalent Si sites. In the first Si site, Si(1) is bonded to six equivalent O(2) atoms to form SiO6 octahedra that share corners with six equivalent Si(2)O4 tetrahedra, edges with six equivalent Na(3)O8 hexagonal bipyramids, and faces with two equivalent Na(1)O6 octahedra. In the second Si site, Si(2) is bonded to one O(2), one O(3), and two equivalent O(1) atoms to form SiO4 tetrahedra that share  a cornercorner with one Na(3)O8 hexagonal bipyramid,  a cornercorner with one Si(1)O6 octahedra, corners with two equivalent Na(1)O6 octahedra, corners with two equivalent Si(2)O4 tetrahedra, and edges with two equivalent Na(3)O8 hexagonal bipyramids. The corner-sharing octahedral tilt angles range from 30-58°. There are three inequivalent O sites. In the first O site, O(1) is bonded to one Na(3), two equivalent Na(2), and two equivalent Si(2) atoms to form a mixture of distorted edge and corner-sharing ONa3Si2 trigonal bipyramids. In the second O site, O(2) is bonded in a 5-coordinate geometry to one Na(1), two equivalent Na(3), one Si(1), and one Si(2) atom. In the third O site, O(3) is bonded in a 5-coordinate geometry to one Na(1), one Na(3), two equivalent Na(2), and one Si(2) atom. is represented by the CIF card [CIF]
data_Na8Si7O18
_symmetry_space_group_name_H-M   'P 1'
_cell_length_a   7.259
_cell_length_b   7.259
_cell_length_c   7.259
_cell_angle_alpha   87.186
_cell_angle_beta   87.186
_cell_angle_gamma   87.186
_symmetry_Int_Tables_number   1
_cell_volume   381.161
_cell_formula_units_Z   1
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
  Na  Na0  1  0.236  0.236  0.236  1.0
  Na  Na1  1  0.764  0.764  0.764  1.0
  Na  Na2  1  0.500  0.500  0.000  1.0
  Na  Na3  1  0.500  0.000  0.500  1.0
  Na  Na4  1  0.000  0.500  0.500  1.0
  Na  Na5  1  0.000  0.000  0.500  1.0
  Na  Na6  1  0.000  0.500  0.000  1.0
  Na  Na7  1  0.500  0.000  0.000  1.0
  Si  Si8  1  0.000  0.000  0.000  1.0
  Si  Si9  1  0.272  0.739  0.272  1.0
  Si  Si10  1  0.739  0.272  0.272  1.0
  Si  Si11  1  0.272  0.272  0.739  1.0
  Si  Si12  1  0.728  0.261  0.728  1.0
  Si  Si13  1  0.261  0.728  0.728  1.0
  Si  Si14  1  0.728  0.728  0.261  1.0
  O  O15  1  0.766  0.500  0.234  1.0
  O  O16  1  0.234  0.766  0.500  1.0
  O  O17  1  0.500  0.234  0.766  1.0
  O  O18  1  0.234  0.500  0.766  1.0
  O  O19  1  0.766  0.234  0.500  1.0
  O  O20  1  0.500  0.766  0.234  1.0
  O  O21  1  0.831  0.831  0.081  1.0
  O  O22  1  0.081  0.831  0.831  1.0
  O  O23  1  0.831  0.081  0.831  1.0
  O  O24  1  0.169  0.169  0.919  1.0
  O  O25  1  0.919  0.169  0.169  1.0
  O  O26  1  0.169  0.919  0.169  1.0
  O  O27  1  0.787  0.787  0.456  1.0
  O  O28  1  0.456  0.787  0.787  1.0
  O  O29  1  0.787  0.456  0.787  1.0
  O  O30  1  0.213  0.544  0.213  1.0
  O  O31  1  0.544  0.213  0.213  1.0
  O  O32  1  0.213  0.213  0.544  1.0
[/CIF]
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