The material structure described by Sr7Nb6O21 crystallizes in the trigonal R-3 space group. There are four inequivalent Sr sites. In the first Sr site, Sr(1) is bonded in a 6-coordinate geometry to six equivalent O(2) atoms. In the second Sr site, Sr(2) is bonded in a 6-coordinate geometry to three equivalent O(1) and three equivalent O(2) atoms. In the third Sr site, Sr(3) is bonded to three equivalent O(1), three equivalent O(4), and six equivalent O(3) atoms to form SrO12 cuboctahedra that share corners with nine equivalent Sr(3)O12 cuboctahedra, faces with three equivalent Sr(4)O12 cuboctahedra,  a faceface with one Nb(2)O6 octahedra, faces with three equivalent Nb(1)O6 octahedra, and faces with four equivalent Nb(3)O6 octahedra. In the fourth Sr site, Sr(4) is bonded to six equivalent O(3) and six equivalent O(4) atoms to form SrO12 cuboctahedra that share corners with six equivalent Sr(4)O12 cuboctahedra, faces with six equivalent Sr(3)O12 cuboctahedra, faces with two equivalent Nb(1)O6 octahedra, and faces with six equivalent Nb(3)O6 octahedra. There are three inequivalent Nb sites. In the first Nb site, Nb(1) is bonded to three equivalent O(1) and three equivalent O(3) atoms to form NbO6 octahedra that share corners with three equivalent Nb(2)O6 octahedra, corners with three equivalent Nb(3)O6 octahedra,  a faceface with one Sr(4)O12 cuboctahedra, and faces with three equivalent Sr(3)O12 cuboctahedra. The corner-sharing octahedral tilt angles range from 9-18°. In the second Nb site, Nb(2) is bonded to three equivalent O(1) and three equivalent O(2) atoms to form distorted NbO6 octahedra that share corners with three equivalent Nb(1)O6 octahedra and  a faceface with one Sr(3)O12 cuboctahedra. The corner-sharing octahedral tilt angles are 18°. In the third Nb site, Nb(3) is bonded to three equivalent O(3) and three equivalent O(4) atoms to form NbO6 octahedra that share corners with three equivalent Nb(1)O6 octahedra, corners with three equivalent Nb(3)O6 octahedra, faces with three equivalent Sr(4)O12 cuboctahedra, and faces with four equivalent Sr(3)O12 cuboctahedra. The corner-sharing octahedral tilt angles range from 0-9°. There are four inequivalent O sites. In the first O site, O(1) is bonded in a 4-coordinate geometry to one Sr(2), one Sr(3), one Nb(1), and one Nb(2) atom. In the second O site, O(2) is bonded in a 4-coordinate geometry to one Sr(2), two equivalent Sr(1), and one Nb(2) atom. In the third O site, O(3) is bonded in a 2-coordinate geometry to one Sr(4), two equivalent Sr(3), one Nb(1), and one Nb(3) atom. In the fourth O site, O(4) is bonded in a distorted linear geometry to two equivalent Sr(3), two equivalent Sr(4), and two equivalent Nb(3) atoms. is represented by the CIF file [CIF]
data_Sr7Nb6O21
_symmetry_space_group_name_H-M   'P 1'
_cell_length_a   16.608
_cell_length_b   16.608
_cell_length_c   16.608
_cell_angle_alpha   19.825
_cell_angle_beta   19.825
_cell_angle_gamma   19.825
_symmetry_Int_Tables_number   1
_cell_volume   460.822
_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
  Sr  Sr0  1  0.847  0.847  0.847  1.0
  Sr  Sr1  1  0.768  0.768  0.768  1.0
  Sr  Sr2  1  0.232  0.232  0.232  1.0
  Sr  Sr3  1  0.153  0.153  0.153  1.0
  Sr  Sr4  1  0.384  0.384  0.384  1.0
  Sr  Sr5  1  0.616  0.616  0.616  1.0
  Sr  Sr6  1  0.000  0.000  0.000  1.0
  Nb  Nb7  1  0.927  0.927  0.927  1.0
  Nb  Nb8  1  0.457  0.457  0.457  1.0
  Nb  Nb9  1  0.309  0.309  0.309  1.0
  Nb  Nb10  1  0.073  0.073  0.073  1.0
  Nb  Nb11  1  0.691  0.691  0.691  1.0
  Nb  Nb12  1  0.543  0.543  0.543  1.0
  O  O13  1  0.754  0.308  0.219  1.0
  O  O14  1  0.707  0.140  0.575  1.0
  O  O15  1  0.575  0.707  0.140  1.0
  O  O16  1  0.626  0.142  0.091  1.0
  O  O17  1  0.140  0.575  0.707  1.0
  O  O18  1  0.246  0.692  0.781  1.0
  O  O19  1  0.781  0.246  0.692  1.0
  O  O20  1  0.308  0.219  0.754  1.0
  O  O21  1  0.909  0.374  0.858  1.0
  O  O22  1  0.293  0.860  0.425  1.0
  O  O23  1  0.500  0.500  0.000  1.0
  O  O24  1  0.142  0.091  0.626  1.0
  O  O25  1  0.425  0.293  0.860  1.0
  O  O26  1  0.374  0.858  0.909  1.0
  O  O27  1  0.219  0.754  0.308  1.0
  O  O28  1  0.860  0.425  0.293  1.0
  O  O29  1  0.091  0.626  0.142  1.0
  O  O30  1  0.858  0.909  0.374  1.0
  O  O31  1  0.692  0.781  0.246  1.0
  O  O32  1  0.500  0.000  0.500  1.0
  O  O33  1  0.000  0.500  0.500  1.0
[/CIF]
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The material structure described by Sr7Nb6O21 crystallizes in the trigonal R-3 space group. There are four inequivalent Sr sites. In the first Sr site, Sr(1) is bonded in a 6-coordinate geometry to six equivalent O(2) atoms. In the second Sr site, Sr(2) is bonded in a 6-coordinate geometry to three equivalent O(1) and three equivalent O(2) atoms. In the third Sr site, Sr(3) is bonded to three equivalent O(1), three equivalent O(4), and six equivalent O(3) atoms to form SrO12 cuboctahedra that share corners with nine equivalent Sr(3)O12 cuboctahedra, faces with three equivalent Sr(4)O12 cuboctahedra,  a faceface with one Nb(2)O6 octahedra, faces with three equivalent Nb(1)O6 octahedra, and faces with four equivalent Nb(3)O6 octahedra. In the fourth Sr site, Sr(4) is bonded to six equivalent O(3) and six equivalent O(4) atoms to form SrO12 cuboctahedra that share corners with six equivalent Sr(4)O12 cuboctahedra, faces with six equivalent Sr(3)O12 cuboctahedra, faces with two equivalent Nb(1)O6 octahedra, and faces with six equivalent Nb(3)O6 octahedra. There are three inequivalent Nb sites. In the first Nb site, Nb(1) is bonded to three equivalent O(1) and three equivalent O(3) atoms to form NbO6 octahedra that share corners with three equivalent Nb(2)O6 octahedra, corners with three equivalent Nb(3)O6 octahedra,  a faceface with one Sr(4)O12 cuboctahedra, and faces with three equivalent Sr(3)O12 cuboctahedra. The corner-sharing octahedral tilt angles range from 9-18°. In the second Nb site, Nb(2) is bonded to three equivalent O(1) and three equivalent O(2) atoms to form distorted NbO6 octahedra that share corners with three equivalent Nb(1)O6 octahedra and  a faceface with one Sr(3)O12 cuboctahedra. The corner-sharing octahedral tilt angles are 18°. In the third Nb site, Nb(3) is bonded to three equivalent O(3) and three equivalent O(4) atoms to form NbO6 octahedra that share corners with three equivalent Nb(1)O6 octahedra, corners with three equivalent Nb(3)O6 octahedra, faces with three equivalent Sr(4)O12 cuboctahedra, and faces with four equivalent Sr(3)O12 cuboctahedra. The corner-sharing octahedral tilt angles range from 0-9°. There are four inequivalent O sites. In the first O site, O(1) is bonded in a 4-coordinate geometry to one Sr(2), one Sr(3), one Nb(1), and one Nb(2) atom. In the second O site, O(2) is bonded in a 4-coordinate geometry to one Sr(2), two equivalent Sr(1), and one Nb(2) atom. In the third O site, O(3) is bonded in a 2-coordinate geometry to one Sr(4), two equivalent Sr(3), one Nb(1), and one Nb(3) atom. In the fourth O site, O(4) is bonded in a distorted linear geometry to two equivalent Sr(3), two equivalent Sr(4), and two equivalent Nb(3) atoms. is represented by the CIF file [CIF]
data_Sr7Nb6O21
_symmetry_space_group_name_H-M   'P 1'
_cell_length_a   16.608
_cell_length_b   16.608
_cell_length_c   16.608
_cell_angle_alpha   19.825
_cell_angle_beta   19.825
_cell_angle_gamma   19.825
_symmetry_Int_Tables_number   1
_cell_volume   460.822
_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
  Sr  Sr0  1  0.847  0.847  0.847  1.0
  Sr  Sr1  1  0.768  0.768  0.768  1.0
  Sr  Sr2  1  0.232  0.232  0.232  1.0
  Sr  Sr3  1  0.153  0.153  0.153  1.0
  Sr  Sr4  1  0.384  0.384  0.384  1.0
  Sr  Sr5  1  0.616  0.616  0.616  1.0
  Sr  Sr6  1  0.000  0.000  0.000  1.0
  Nb  Nb7  1  0.927  0.927  0.927  1.0
  Nb  Nb8  1  0.457  0.457  0.457  1.0
  Nb  Nb9  1  0.309  0.309  0.309  1.0
  Nb  Nb10  1  0.073  0.073  0.073  1.0
  Nb  Nb11  1  0.691  0.691  0.691  1.0
  Nb  Nb12  1  0.543  0.543  0.543  1.0
  O  O13  1  0.754  0.308  0.219  1.0
  O  O14  1  0.707  0.140  0.575  1.0
  O  O15  1  0.575  0.707  0.140  1.0
  O  O16  1  0.626  0.142  0.091  1.0
  O  O17  1  0.140  0.575  0.707  1.0
  O  O18  1  0.246  0.692  0.781  1.0
  O  O19  1  0.781  0.246  0.692  1.0
  O  O20  1  0.308  0.219  0.754  1.0
  O  O21  1  0.909  0.374  0.858  1.0
  O  O22  1  0.293  0.860  0.425  1.0
  O  O23  1  0.500  0.500  0.000  1.0
  O  O24  1  0.142  0.091  0.626  1.0
  O  O25  1  0.425  0.293  0.860  1.0
  O  O26  1  0.374  0.858  0.909  1.0
  O  O27  1  0.219  0.754  0.308  1.0
  O  O28  1  0.860  0.425  0.293  1.0
  O  O29  1  0.091  0.626  0.142  1.0
  O  O30  1  0.858  0.909  0.374  1.0
  O  O31  1  0.692  0.781  0.246  1.0
  O  O32  1  0.500  0.000  0.500  1.0
  O  O33  1  0.000  0.500  0.500  1.0
[/CIF]
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