The material described by Sr6Nb7O21 crystallizes in the triclinic P1 space group. There are six inequivalent Sr sites. In the first Sr site, Sr(1) is bonded to one O(10), one O(11), one O(15), one O(16), one O(17), one O(2), one O(21), one O(3), one O(4), one O(5), one O(6), and one O(9) atom to form SrO12 cuboctahedra that share corners with two equivalent Sr(2)O12 cuboctahedra, corners with two equivalent Sr(3)O12 cuboctahedra, corners with two equivalent Sr(4)O12 cuboctahedra, corners with two equivalent Sr(5)O12 cuboctahedra, corners with two equivalent Sr(6)O12 cuboctahedra,  a faceface with one Sr(2)O12 cuboctahedra,  a faceface with one Sr(3)O12 cuboctahedra,  a faceface with one Sr(4)O12 cuboctahedra,  a faceface with one Sr(5)O12 cuboctahedra,  a faceface with one Sr(6)O12 cuboctahedra,  a faceface with one Nb(2)O6 octahedra,  a faceface with one Nb(3)O6 octahedra,  a faceface with one Nb(4)O6 octahedra,  a faceface with one Nb(5)O6 octahedra,  a faceface with one Nb(6)O6 octahedra,  a faceface with one Nb(7)O6 octahedra, and faces with two equivalent Nb(1)O6 octahedra. In the second Sr site, Sr(2) is bonded to one O(1), one O(11), one O(13), one O(14), one O(17), one O(19), one O(4), one O(5), one O(6), one O(7), one O(8), and one O(9) atom to form SrO12 cuboctahedra that share corners with two equivalent Sr(1)O12 cuboctahedra, corners with two equivalent Sr(3)O12 cuboctahedra, corners with two equivalent Sr(4)O12 cuboctahedra, corners with two equivalent Sr(5)O12 cuboctahedra, corners with two equivalent Sr(6)O12 cuboctahedra,  a faceface with one Sr(1)O12 cuboctahedra,  a faceface with one Sr(3)O12 cuboctahedra,  a faceface with one Sr(4)O12 cuboctahedra,  a faceface with one Sr(5)O12 cuboctahedra,  a faceface with one Sr(6)O12 cuboctahedra,  a faceface with one Nb(1)O6 octahedra,  a faceface with one Nb(3)O6 octahedra,  a faceface with one Nb(4)O6 octahedra,  a faceface with one Nb(5)O6 octahedra,  a faceface with one Nb(6)O6 octahedra,  a faceface with one Nb(7)O6 octahedra, and faces with two equivalent Nb(2)O6 octahedra. In the third Sr site, Sr(3) is bonded to one O(1), one O(10), one O(11), one O(12), one O(14), one O(16), one O(18), one O(2), one O(3), one O(7), one O(8), and one O(9) atom to form SrO12 cuboctahedra that share corners with two equivalent Sr(1)O12 cuboctahedra, corners with two equivalent Sr(2)O12 cuboctahedra, corners with two equivalent Sr(4)O12 cuboctahedra, corners with two equivalent Sr(5)O12 cuboctahedra, corners with two equivalent Sr(6)O12 cuboctahedra,  a faceface with one Sr(1)O12 cuboctahedra,  a faceface with one Sr(2)O12 cuboctahedra,  a faceface with one Sr(4)O12 cuboctahedra,  a faceface with one Sr(5)O12 cuboctahedra,  a faceface with one Sr(6)O12 cuboctahedra,  a faceface with one Nb(1)O6 octahedra,  a faceface with one Nb(2)O6 octahedra,  a faceface with one Nb(4)O6 octahedra,  a faceface with one Nb(5)O6 octahedra,  a faceface with one Nb(6)O6 octahedra,  a faceface with one Nb(7)O6 octahedra, and faces with two equivalent Nb(3)O6 octahedra. In the fourth Sr site, Sr(4) is bonded to one O(10), one O(11), one O(12), one O(13), one O(14), one O(15), one O(18), one O(19), one O(20), one O(3), one O(4), and one O(5) atom to form SrO12 cuboctahedra that share corners with two equivalent Sr(1)O12 cuboctahedra, corners with two equivalent Sr(2)O12 cuboctahedra, corners with two equivalent Sr(3)O12 cuboctahedra, corners with two equivalent Sr(5)O12 cuboctahedra, corners with two equivalent Sr(6)O12 cuboctahedra,  a faceface with one Sr(1)O12 cuboctahedra,  a faceface with one Sr(2)O12 cuboctahedra,  a faceface with one Sr(3)O12 cuboctahedra,  a faceface with one Sr(5)O12 cuboctahedra,  a faceface with one Sr(6)O12 cuboctahedra,  a faceface with one Nb(1)O6 octahedra,  a faceface with one Nb(2)O6 octahedra,  a faceface with one Nb(3)O6 octahedra,  a faceface with one Nb(5)O6 octahedra,  a faceface with one Nb(6)O6 octahedra,  a faceface with one Nb(7)O6 octahedra, and faces with two equivalent Nb(4)O6 octahedra. In the fifth Sr site, Sr(5) is bonded to one O(13), one O(14), one O(15), one O(16), one O(17), one O(18), one O(2), one O(20), one O(21), one O(5), one O(7), and one O(8) atom to form SrO12 cuboctahedra that share corners with two equivalent Sr(1)O12 cuboctahedra, corners with two equivalent Sr(2)O12 cuboctahedra, corners with two equivalent Sr(3)O12 cuboctahedra, corners with two equivalent Sr(4)O12 cuboctahedra, corners with two equivalent Sr(6)O12 cuboctahedra,  a faceface with one Sr(1)O12 cuboctahedra,  a faceface with one Sr(2)O12 cuboctahedra,  a faceface with one Sr(3)O12 cuboctahedra,  a faceface with one Sr(4)O12 cuboctahedra,  a faceface with one Sr(6)O12 cuboctahedra,  a faceface with one Nb(1)O6 octahedra,  a faceface with one Nb(2)O6 octahedra,  a faceface with one Nb(3)O6 octahedra,  a faceface with one Nb(4)O6 octahedra,  a faceface with one Nb(6)O6 octahedra,  a faceface with one Nb(7)O6 octahedra, and faces with two equivalent Nb(5)O6 octahedra. In the sixth Sr site, Sr(6) is bonded to one O(1), one O(10), one O(12), one O(16), one O(17), one O(18), one O(19), one O(20), one O(21), one O(4), one O(6), and one O(8) atom to form SrO12 cuboctahedra that share corners with two equivalent Sr(1)O12 cuboctahedra, corners with two equivalent Sr(2)O12 cuboctahedra, corners with two equivalent Sr(3)O12 cuboctahedra, corners with two equivalent Sr(4)O12 cuboctahedra, corners with two equivalent Sr(5)O12 cuboctahedra,  a faceface with one Sr(1)O12 cuboctahedra,  a faceface with one Sr(2)O12 cuboctahedra,  a faceface with one Sr(3)O12 cuboctahedra,  a faceface with one Sr(4)O12 cuboctahedra,  a faceface with one Sr(5)O12 cuboctahedra,  a faceface with one Nb(1)O6 octahedra,  a faceface with one Nb(2)O6 octahedra,  a faceface with one Nb(3)O6 octahedra,  a faceface with one Nb(4)O6 octahedra,  a faceface with one Nb(5)O6 octahedra,  a faceface with one Nb(7)O6 octahedra, and faces with two equivalent Nb(6)O6 octahedra. There are seven inequivalent Nb sites. In the first Nb site, Nb(1) is bonded to one O(11), one O(17), one O(2), one O(21), one O(3), and one O(4) atom to form NbO6 octahedra that share  a cornercorner with one Nb(2)O6 octahedra,  a cornercorner with one Nb(3)O6 octahedra,  a cornercorner with one Nb(4)O6 octahedra,  a cornercorner with one Nb(5)O6 octahedra,  a cornercorner with one Nb(6)O6 octahedra,  a cornercorner with one Nb(7)O6 octahedra,  a faceface with one Sr(2)O12 cuboctahedra,  a faceface with one Sr(3)O12 cuboctahedra,  a faceface with one Sr(4)O12 cuboctahedra,  a faceface with one Sr(5)O12 cuboctahedra,  a faceface with one Sr(6)O12 cuboctahedra, and faces with two equivalent Sr(1)O12 cuboctahedra. The corner-sharing octahedral tilt angles range from 1-7°. In the second Nb site, Nb(2) is bonded to one O(1), one O(14), one O(4), one O(5), one O(6), and one O(7) atom to form NbO6 octahedra that share  a cornercorner with one Nb(1)O6 octahedra,  a cornercorner with one Nb(3)O6 octahedra,  a cornercorner with one Nb(4)O6 octahedra,  a cornercorner with one Nb(5)O6 octahedra,  a cornercorner with one Nb(6)O6 octahedra,  a cornercorner with one Nb(7)O6 octahedra,  a faceface with one Sr(1)O12 cuboctahedra,  a faceface with one Sr(3)O12 cuboctahedra,  a faceface with one Sr(4)O12 cuboctahedra,  a faceface with one Sr(5)O12 cuboctahedra,  a faceface with one Sr(6)O12 cuboctahedra, and faces with two equivalent Sr(2)O12 cuboctahedra. The corner-sharing octahedral tilt angles range from 1-6°. In the third Nb site, Nb(3) is bonded to one O(10), one O(18), one O(3), one O(7), one O(8), and one O(9) atom to form NbO6 octahedra that share  a cornercorner with one Nb(1)O6 octahedra,  a cornercorner with one Nb(2)O6 octahedra,  a cornercorner with one Nb(4)O6 octahedra,  a cornercorner with one Nb(5)O6 octahedra,  a cornercorner with one Nb(6)O6 octahedra,  a cornercorner with one Nb(7)O6 octahedra,  a faceface with one Sr(1)O12 cuboctahedra,  a faceface with one Sr(2)O12 cuboctahedra,  a faceface with one Sr(4)O12 cuboctahedra,  a faceface with one Sr(5)O12 cuboctahedra,  a faceface with one Sr(6)O12 cuboctahedra, and faces with two equivalent Sr(3)O12 cuboctahedra. The corner-sharing octahedral tilt angles range from 1-7°. In the fourth Nb site, Nb(4) is bonded to one O(10), one O(11), one O(12), one O(13), one O(20), and one O(5) atom to form NbO6 octahedra that share  a cornercorner with one Nb(1)O6 octahedra,  a cornercorner with one Nb(2)O6 octahedra,  a cornercorner with one Nb(3)O6 octahedra,  a cornercorner with one Nb(5)O6 octahedra,  a cornercorner with one Nb(6)O6 octahedra,  a cornercorner with one Nb(7)O6 octahedra,  a faceface with one Sr(1)O12 cuboctahedra,  a faceface with one Sr(2)O12 cuboctahedra,  a faceface with one Sr(3)O12 cuboctahedra,  a faceface with one Sr(5)O12 cuboctahedra,  a faceface with one Sr(6)O12 cuboctahedra, and faces with two equivalent Sr(4)O12 cuboctahedra. The corner-sharing octahedral tilt angles range from 1-7°. In the fifth Nb site, Nb(5) is bonded to one O(13), one O(14), one O(15), one O(16), one O(21), and one O(8) atom to form NbO6 octahedra that share  a cornercorner with one Nb(1)O6 octahedra,  a cornercorner with one Nb(2)O6 octahedra,  a cornercorner with one Nb(3)O6 octahedra,  a cornercorner with one Nb(4)O6 octahedra,  a cornercorner with one Nb(6)O6 octahedra,  a cornercorner with one Nb(7)O6 octahedra,  a faceface with one Sr(1)O12 cuboctahedra,  a faceface with one Sr(2)O12 cuboctahedra,  a faceface with one Sr(3)O12 cuboctahedra,  a faceface with one Sr(4)O12 cuboctahedra,  a faceface with one Sr(6)O12 cuboctahedra, and faces with two equivalent Sr(5)O12 cuboctahedra. The corner-sharing octahedral tilt angles range from 1-6°. In the sixth Nb site, Nb(6) is bonded to one O(12), one O(16), one O(17), one O(18), one O(19), and one O(6) atom to form NbO6 octahedra that share  a cornercorner with one Nb(1)O6 octahedra,  a cornercorner with one Nb(2)O6 octahedra,  a cornercorner with one Nb(3)O6 octahedra,  a cornercorner with one Nb(4)O6 octahedra,  a cornercorner with one Nb(5)O6 octahedra,  a cornercorner with one Nb(7)O6 octahedra,  a faceface with one Sr(1)O12 cuboctahedra,  a faceface with one Sr(2)O12 cuboctahedra,  a faceface with one Sr(3)O12 cuboctahedra,  a faceface with one Sr(4)O12 cuboctahedra,  a faceface with one Sr(5)O12 cuboctahedra, and faces with two equivalent Sr(6)O12 cuboctahedra. The corner-sharing octahedral tilt angles range from 1-7°. In the seventh Nb site, Nb(7) is bonded to one O(1), one O(15), one O(19), one O(2), one O(20), and one O(9) atom to form NbO6 octahedra that share  a cornercorner with one Nb(1)O6 octahedra,  a cornercorner with one Nb(2)O6 octahedra,  a cornercorner with one Nb(3)O6 octahedra,  a cornercorner with one Nb(4)O6 octahedra,  a cornercorner with one Nb(5)O6 octahedra,  a cornercorner with one Nb(6)O6 octahedra,  a faceface with one Sr(1)O12 cuboctahedra,  a faceface with one Sr(2)O12 cuboctahedra,  a faceface with one Sr(3)O12 cuboctahedra,  a faceface with one Sr(4)O12 cuboctahedra,  a faceface with one Sr(5)O12 cuboctahedra, and  a faceface with one Sr(6)O12 cuboctahedra. The corner-sharing octahedral tilt angles range from 4-6°. There are twenty-one inequivalent O sites. In the first O site, O(1) is bonded to one Sr(2), one Sr(3), one Sr(6), one Nb(2), and one Nb(7) atom to form a mixture of distorted face, corner, and edge-sharing OSr3Nb2 square pyramids. In the second O site, O(2) is bonded to one Sr(1), one Sr(3), one Sr(5), one Nb(1), and one Nb(7) atom to form a mixture of distorted face, corner, and edge-sharing OSr3Nb2 square pyramids. In the third O site, O(3) is bonded to one Sr(1), one Sr(3), one Sr(4), one Nb(1), and one Nb(3) atom to form a mixture of distorted corner and edge-sharing OSr3Nb2 square pyramids. In the fourth O site, O(4) is bonded in a distorted linear geometry to one Sr(1), one Sr(2), one Sr(4), one Sr(6), one Nb(1), and one Nb(2) atom. In the fifth O site, O(5) is bonded in a distorted linear geometry to one Sr(1), one Sr(2), one Sr(4), one Sr(5), one Nb(2), and one Nb(4) atom. In the sixth O site, O(6) is bonded in a distorted linear geometry to one Sr(1), one Sr(2), one Sr(6), one Nb(2), and one Nb(6) atom. In the seventh O site, O(7) is bonded in a distorted linear geometry to one Sr(2), one Sr(3), one Sr(5), one Nb(2), and one Nb(3) atom. In the eighth O site, O(8) is bonded in a distorted linear geometry to one Sr(2), one Sr(3), one Sr(5), one Sr(6), one Nb(3), and one Nb(5) atom. In the ninth O site, O(9) is bonded in a distorted linear geometry to one Sr(1), one Sr(2), one Sr(3), one Nb(3), and one Nb(7) atom. In the tenth O site, O(10) is bonded in a distorted linear geometry to one Sr(1), one Sr(3), one Sr(4), one Sr(6), one Nb(3), and one Nb(4) atom. In the eleventh O site, O(11) is bonded in a distorted linear geometry to one Sr(1), one Sr(2), one Sr(3), one Sr(4), one Nb(1), and one Nb(4) atom. In the twelfth O site, O(12) is bonded to one Sr(3), one Sr(4), one Sr(6), one Nb(4), and one Nb(6) atom to form a mixture of distorted corner and edge-sharing OSr3Nb2 square pyramids. In the thirteenth O site, O(13) is bonded to one Sr(2), one Sr(4), one Sr(5), one Nb(4), and one Nb(5) atom to form a mixture of distorted corner and edge-sharing OSr3Nb2 square pyramids. In the fourteenth O site, O(14) is bonded in a distorted linear geometry to one Sr(2), one Sr(3), one Sr(4), one Sr(5), one Nb(2), and one Nb(5) atom. In the fifteenth O site, O(15) is bonded to one Sr(1), one Sr(4), one Sr(5), one Nb(5), and one Nb(7) atom to form a mixture of distorted face, corner, and edge-sharing OSr3Nb2 square pyramids. In the sixteenth O site, O(16) is bonded in a distorted linear geometry to one Sr(1), one Sr(3), one Sr(5), one Sr(6), one Nb(5), and one Nb(6) atom. In the seventeenth O site, O(17) is bonded in a distorted linear geometry to one Sr(1), one Sr(2), one Sr(5), one Sr(6), one Nb(1), and one Nb(6) atom. In the eighteenth O site, O(18) is bonded in a distorted linear geometry to one Sr(3), one Sr(4), one Sr(5), one Sr(6), one Nb(3), and one Nb(6) atom. In the nineteenth O site, O(19) is bonded to one Sr(2), one Sr(4), one Sr(6), one Nb(6), and one Nb(7) atom to form a mixture of distorted face, corner, and edge-sharing OSr3Nb2 square pyramids. In the twentieth O site, O(20) is bonded to one Sr(4), one Sr(5), one Sr(6), one Nb(4), and one Nb(7) atom to form a mixture of distorted face, corner, and edge-sharing OSr3Nb2 square pyramids. In the twenty-first O site, O(21) is bonded in a distorted linear geometry to one Sr(1), one Sr(5), one Sr(6), one Nb(1), and one Nb(5) atom. is represented by the CIF file [CIF]
data_Sr6Nb7O21
_symmetry_space_group_name_H-M   'P 1'
_cell_length_a   7.051
_cell_length_b   9.100
_cell_length_c   9.101
_cell_angle_alpha   113.566
_cell_angle_beta   104.963
_cell_angle_gamma   104.967
_symmetry_Int_Tables_number   1
_cell_volume   472.084
_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  0.563  0.280  0.421  1.0
  Sr  Sr1  1  0.142  0.572  0.860  1.0
  Sr  Sr2  1  0.713  0.855  0.284  1.0
  Sr  Sr3  1  0.287  0.144  0.718  1.0
  Sr  Sr4  1  0.851  0.422  0.140  1.0
  Sr  Sr5  1  0.428  0.716  0.573  1.0
  Nb  Nb6  1  0.069  0.284  0.426  1.0
  Nb  Nb7  1  0.644  0.576  0.858  1.0
  Nb  Nb8  1  0.210  0.859  0.283  1.0
  Nb  Nb9  1  0.786  0.142  0.717  1.0
  Nb  Nb10  1  0.353  0.425  0.142  1.0
  Nb  Nb11  1  0.930  0.718  0.575  1.0
  Nb  Nb12  1  0.499  1.000  1.000  1.0
  O  O13  1  0.558  0.780  0.927  1.0
  O  O14  1  0.779  0.147  0.221  1.0
  O  O15  1  0.143  0.073  0.369  1.0
  O  O16  1  0.357  0.429  0.643  1.0
  O  O17  1  0.712  0.358  0.788  1.0
  O  O18  1  0.778  0.634  0.706  1.0
  O  O19  1  0.925  0.708  0.073  1.0
  O  O20  1  0.285  0.644  0.213  1.0
  O  O21  1  0.363  0.929  0.147  1.0
  O  O22  1  0.500  0.001  0.499  1.0
  O  O23  1  0.929  0.221  0.576  1.0
  O  O24  1  0.854  0.928  0.631  1.0
  O  O25  1  0.071  0.293  0.928  1.0
  O  O26  1  0.496  0.500  0.998  1.0
  O  O27  1  0.440  0.221  0.072  1.0
  O  O28  1  0.644  0.573  0.355  1.0
  O  O29  1  0.000  0.501  0.498  1.0
  O  O30  1  0.072  0.785  0.428  1.0
  O  O31  1  0.219  0.854  0.779  1.0
  O  O32  1  0.633  0.072  0.853  1.0
  O  O33  1  0.222  0.365  0.294  1.0
[/CIF]
.
The material described by Sr6Nb7O21 crystallizes in the triclinic P1 space group. There are six inequivalent Sr sites. In the first Sr site, Sr(1) is bonded to one O(10), one O(11), one O(15), one O(16), one O(17), one O(2), one O(21), one O(3), one O(4), one O(5), one O(6), and one O(9) atom to form SrO12 cuboctahedra that share corners with two equivalent Sr(2)O12 cuboctahedra, corners with two equivalent Sr(3)O12 cuboctahedra, corners with two equivalent Sr(4)O12 cuboctahedra, corners with two equivalent Sr(5)O12 cuboctahedra, corners with two equivalent Sr(6)O12 cuboctahedra,  a faceface with one Sr(2)O12 cuboctahedra,  a faceface with one Sr(3)O12 cuboctahedra,  a faceface with one Sr(4)O12 cuboctahedra,  a faceface with one Sr(5)O12 cuboctahedra,  a faceface with one Sr(6)O12 cuboctahedra,  a faceface with one Nb(2)O6 octahedra,  a faceface with one Nb(3)O6 octahedra,  a faceface with one Nb(4)O6 octahedra,  a faceface with one Nb(5)O6 octahedra,  a faceface with one Nb(6)O6 octahedra,  a faceface with one Nb(7)O6 octahedra, and faces with two equivalent Nb(1)O6 octahedra. In the second Sr site, Sr(2) is bonded to one O(1), one O(11), one O(13), one O(14), one O(17), one O(19), one O(4), one O(5), one O(6), one O(7), one O(8), and one O(9) atom to form SrO12 cuboctahedra that share corners with two equivalent Sr(1)O12 cuboctahedra, corners with two equivalent Sr(3)O12 cuboctahedra, corners with two equivalent Sr(4)O12 cuboctahedra, corners with two equivalent Sr(5)O12 cuboctahedra, corners with two equivalent Sr(6)O12 cuboctahedra,  a faceface with one Sr(1)O12 cuboctahedra,  a faceface with one Sr(3)O12 cuboctahedra,  a faceface with one Sr(4)O12 cuboctahedra,  a faceface with one Sr(5)O12 cuboctahedra,  a faceface with one Sr(6)O12 cuboctahedra,  a faceface with one Nb(1)O6 octahedra,  a faceface with one Nb(3)O6 octahedra,  a faceface with one Nb(4)O6 octahedra,  a faceface with one Nb(5)O6 octahedra,  a faceface with one Nb(6)O6 octahedra,  a faceface with one Nb(7)O6 octahedra, and faces with two equivalent Nb(2)O6 octahedra. In the third Sr site, Sr(3) is bonded to one O(1), one O(10), one O(11), one O(12), one O(14), one O(16), one O(18), one O(2), one O(3), one O(7), one O(8), and one O(9) atom to form SrO12 cuboctahedra that share corners with two equivalent Sr(1)O12 cuboctahedra, corners with two equivalent Sr(2)O12 cuboctahedra, corners with two equivalent Sr(4)O12 cuboctahedra, corners with two equivalent Sr(5)O12 cuboctahedra, corners with two equivalent Sr(6)O12 cuboctahedra,  a faceface with one Sr(1)O12 cuboctahedra,  a faceface with one Sr(2)O12 cuboctahedra,  a faceface with one Sr(4)O12 cuboctahedra,  a faceface with one Sr(5)O12 cuboctahedra,  a faceface with one Sr(6)O12 cuboctahedra,  a faceface with one Nb(1)O6 octahedra,  a faceface with one Nb(2)O6 octahedra,  a faceface with one Nb(4)O6 octahedra,  a faceface with one Nb(5)O6 octahedra,  a faceface with one Nb(6)O6 octahedra,  a faceface with one Nb(7)O6 octahedra, and faces with two equivalent Nb(3)O6 octahedra. In the fourth Sr site, Sr(4) is bonded to one O(10), one O(11), one O(12), one O(13), one O(14), one O(15), one O(18), one O(19), one O(20), one O(3), one O(4), and one O(5) atom to form SrO12 cuboctahedra that share corners with two equivalent Sr(1)O12 cuboctahedra, corners with two equivalent Sr(2)O12 cuboctahedra, corners with two equivalent Sr(3)O12 cuboctahedra, corners with two equivalent Sr(5)O12 cuboctahedra, corners with two equivalent Sr(6)O12 cuboctahedra,  a faceface with one Sr(1)O12 cuboctahedra,  a faceface with one Sr(2)O12 cuboctahedra,  a faceface with one Sr(3)O12 cuboctahedra,  a faceface with one Sr(5)O12 cuboctahedra,  a faceface with one Sr(6)O12 cuboctahedra,  a faceface with one Nb(1)O6 octahedra,  a faceface with one Nb(2)O6 octahedra,  a faceface with one Nb(3)O6 octahedra,  a faceface with one Nb(5)O6 octahedra,  a faceface with one Nb(6)O6 octahedra,  a faceface with one Nb(7)O6 octahedra, and faces with two equivalent Nb(4)O6 octahedra. In the fifth Sr site, Sr(5) is bonded to one O(13), one O(14), one O(15), one O(16), one O(17), one O(18), one O(2), one O(20), one O(21), one O(5), one O(7), and one O(8) atom to form SrO12 cuboctahedra that share corners with two equivalent Sr(1)O12 cuboctahedra, corners with two equivalent Sr(2)O12 cuboctahedra, corners with two equivalent Sr(3)O12 cuboctahedra, corners with two equivalent Sr(4)O12 cuboctahedra, corners with two equivalent Sr(6)O12 cuboctahedra,  a faceface with one Sr(1)O12 cuboctahedra,  a faceface with one Sr(2)O12 cuboctahedra,  a faceface with one Sr(3)O12 cuboctahedra,  a faceface with one Sr(4)O12 cuboctahedra,  a faceface with one Sr(6)O12 cuboctahedra,  a faceface with one Nb(1)O6 octahedra,  a faceface with one Nb(2)O6 octahedra,  a faceface with one Nb(3)O6 octahedra,  a faceface with one Nb(4)O6 octahedra,  a faceface with one Nb(6)O6 octahedra,  a faceface with one Nb(7)O6 octahedra, and faces with two equivalent Nb(5)O6 octahedra. In the sixth Sr site, Sr(6) is bonded to one O(1), one O(10), one O(12), one O(16), one O(17), one O(18), one O(19), one O(20), one O(21), one O(4), one O(6), and one O(8) atom to form SrO12 cuboctahedra that share corners with two equivalent Sr(1)O12 cuboctahedra, corners with two equivalent Sr(2)O12 cuboctahedra, corners with two equivalent Sr(3)O12 cuboctahedra, corners with two equivalent Sr(4)O12 cuboctahedra, corners with two equivalent Sr(5)O12 cuboctahedra,  a faceface with one Sr(1)O12 cuboctahedra,  a faceface with one Sr(2)O12 cuboctahedra,  a faceface with one Sr(3)O12 cuboctahedra,  a faceface with one Sr(4)O12 cuboctahedra,  a faceface with one Sr(5)O12 cuboctahedra,  a faceface with one Nb(1)O6 octahedra,  a faceface with one Nb(2)O6 octahedra,  a faceface with one Nb(3)O6 octahedra,  a faceface with one Nb(4)O6 octahedra,  a faceface with one Nb(5)O6 octahedra,  a faceface with one Nb(7)O6 octahedra, and faces with two equivalent Nb(6)O6 octahedra. There are seven inequivalent Nb sites. In the first Nb site, Nb(1) is bonded to one O(11), one O(17), one O(2), one O(21), one O(3), and one O(4) atom to form NbO6 octahedra that share  a cornercorner with one Nb(2)O6 octahedra,  a cornercorner with one Nb(3)O6 octahedra,  a cornercorner with one Nb(4)O6 octahedra,  a cornercorner with one Nb(5)O6 octahedra,  a cornercorner with one Nb(6)O6 octahedra,  a cornercorner with one Nb(7)O6 octahedra,  a faceface with one Sr(2)O12 cuboctahedra,  a faceface with one Sr(3)O12 cuboctahedra,  a faceface with one Sr(4)O12 cuboctahedra,  a faceface with one Sr(5)O12 cuboctahedra,  a faceface with one Sr(6)O12 cuboctahedra, and faces with two equivalent Sr(1)O12 cuboctahedra. The corner-sharing octahedral tilt angles range from 1-7°. In the second Nb site, Nb(2) is bonded to one O(1), one O(14), one O(4), one O(5), one O(6), and one O(7) atom to form NbO6 octahedra that share  a cornercorner with one Nb(1)O6 octahedra,  a cornercorner with one Nb(3)O6 octahedra,  a cornercorner with one Nb(4)O6 octahedra,  a cornercorner with one Nb(5)O6 octahedra,  a cornercorner with one Nb(6)O6 octahedra,  a cornercorner with one Nb(7)O6 octahedra,  a faceface with one Sr(1)O12 cuboctahedra,  a faceface with one Sr(3)O12 cuboctahedra,  a faceface with one Sr(4)O12 cuboctahedra,  a faceface with one Sr(5)O12 cuboctahedra,  a faceface with one Sr(6)O12 cuboctahedra, and faces with two equivalent Sr(2)O12 cuboctahedra. The corner-sharing octahedral tilt angles range from 1-6°. In the third Nb site, Nb(3) is bonded to one O(10), one O(18), one O(3), one O(7), one O(8), and one O(9) atom to form NbO6 octahedra that share  a cornercorner with one Nb(1)O6 octahedra,  a cornercorner with one Nb(2)O6 octahedra,  a cornercorner with one Nb(4)O6 octahedra,  a cornercorner with one Nb(5)O6 octahedra,  a cornercorner with one Nb(6)O6 octahedra,  a cornercorner with one Nb(7)O6 octahedra,  a faceface with one Sr(1)O12 cuboctahedra,  a faceface with one Sr(2)O12 cuboctahedra,  a faceface with one Sr(4)O12 cuboctahedra,  a faceface with one Sr(5)O12 cuboctahedra,  a faceface with one Sr(6)O12 cuboctahedra, and faces with two equivalent Sr(3)O12 cuboctahedra. The corner-sharing octahedral tilt angles range from 1-7°. In the fourth Nb site, Nb(4) is bonded to one O(10), one O(11), one O(12), one O(13), one O(20), and one O(5) atom to form NbO6 octahedra that share  a cornercorner with one Nb(1)O6 octahedra,  a cornercorner with one Nb(2)O6 octahedra,  a cornercorner with one Nb(3)O6 octahedra,  a cornercorner with one Nb(5)O6 octahedra,  a cornercorner with one Nb(6)O6 octahedra,  a cornercorner with one Nb(7)O6 octahedra,  a faceface with one Sr(1)O12 cuboctahedra,  a faceface with one Sr(2)O12 cuboctahedra,  a faceface with one Sr(3)O12 cuboctahedra,  a faceface with one Sr(5)O12 cuboctahedra,  a faceface with one Sr(6)O12 cuboctahedra, and faces with two equivalent Sr(4)O12 cuboctahedra. The corner-sharing octahedral tilt angles range from 1-7°. In the fifth Nb site, Nb(5) is bonded to one O(13), one O(14), one O(15), one O(16), one O(21), and one O(8) atom to form NbO6 octahedra that share  a cornercorner with one Nb(1)O6 octahedra,  a cornercorner with one Nb(2)O6 octahedra,  a cornercorner with one Nb(3)O6 octahedra,  a cornercorner with one Nb(4)O6 octahedra,  a cornercorner with one Nb(6)O6 octahedra,  a cornercorner with one Nb(7)O6 octahedra,  a faceface with one Sr(1)O12 cuboctahedra,  a faceface with one Sr(2)O12 cuboctahedra,  a faceface with one Sr(3)O12 cuboctahedra,  a faceface with one Sr(4)O12 cuboctahedra,  a faceface with one Sr(6)O12 cuboctahedra, and faces with two equivalent Sr(5)O12 cuboctahedra. The corner-sharing octahedral tilt angles range from 1-6°. In the sixth Nb site, Nb(6) is bonded to one O(12), one O(16), one O(17), one O(18), one O(19), and one O(6) atom to form NbO6 octahedra that share  a cornercorner with one Nb(1)O6 octahedra,  a cornercorner with one Nb(2)O6 octahedra,  a cornercorner with one Nb(3)O6 octahedra,  a cornercorner with one Nb(4)O6 octahedra,  a cornercorner with one Nb(5)O6 octahedra,  a cornercorner with one Nb(7)O6 octahedra,  a faceface with one Sr(1)O12 cuboctahedra,  a faceface with one Sr(2)O12 cuboctahedra,  a faceface with one Sr(3)O12 cuboctahedra,  a faceface with one Sr(4)O12 cuboctahedra,  a faceface with one Sr(5)O12 cuboctahedra, and faces with two equivalent Sr(6)O12 cuboctahedra. The corner-sharing octahedral tilt angles range from 1-7°. In the seventh Nb site, Nb(7) is bonded to one O(1), one O(15), one O(19), one O(2), one O(20), and one O(9) atom to form NbO6 octahedra that share  a cornercorner with one Nb(1)O6 octahedra,  a cornercorner with one Nb(2)O6 octahedra,  a cornercorner with one Nb(3)O6 octahedra,  a cornercorner with one Nb(4)O6 octahedra,  a cornercorner with one Nb(5)O6 octahedra,  a cornercorner with one Nb(6)O6 octahedra,  a faceface with one Sr(1)O12 cuboctahedra,  a faceface with one Sr(2)O12 cuboctahedra,  a faceface with one Sr(3)O12 cuboctahedra,  a faceface with one Sr(4)O12 cuboctahedra,  a faceface with one Sr(5)O12 cuboctahedra, and  a faceface with one Sr(6)O12 cuboctahedra. The corner-sharing octahedral tilt angles range from 4-6°. There are twenty-one inequivalent O sites. In the first O site, O(1) is bonded to one Sr(2), one Sr(3), one Sr(6), one Nb(2), and one Nb(7) atom to form a mixture of distorted face, corner, and edge-sharing OSr3Nb2 square pyramids. In the second O site, O(2) is bonded to one Sr(1), one Sr(3), one Sr(5), one Nb(1), and one Nb(7) atom to form a mixture of distorted face, corner, and edge-sharing OSr3Nb2 square pyramids. In the third O site, O(3) is bonded to one Sr(1), one Sr(3), one Sr(4), one Nb(1), and one Nb(3) atom to form a mixture of distorted corner and edge-sharing OSr3Nb2 square pyramids. In the fourth O site, O(4) is bonded in a distorted linear geometry to one Sr(1), one Sr(2), one Sr(4), one Sr(6), one Nb(1), and one Nb(2) atom. In the fifth O site, O(5) is bonded in a distorted linear geometry to one Sr(1), one Sr(2), one Sr(4), one Sr(5), one Nb(2), and one Nb(4) atom. In the sixth O site, O(6) is bonded in a distorted linear geometry to one Sr(1), one Sr(2), one Sr(6), one Nb(2), and one Nb(6) atom. In the seventh O site, O(7) is bonded in a distorted linear geometry to one Sr(2), one Sr(3), one Sr(5), one Nb(2), and one Nb(3) atom. In the eighth O site, O(8) is bonded in a distorted linear geometry to one Sr(2), one Sr(3), one Sr(5), one Sr(6), one Nb(3), and one Nb(5) atom. In the ninth O site, O(9) is bonded in a distorted linear geometry to one Sr(1), one Sr(2), one Sr(3), one Nb(3), and one Nb(7) atom. In the tenth O site, O(10) is bonded in a distorted linear geometry to one Sr(1), one Sr(3), one Sr(4), one Sr(6), one Nb(3), and one Nb(4) atom. In the eleventh O site, O(11) is bonded in a distorted linear geometry to one Sr(1), one Sr(2), one Sr(3), one Sr(4), one Nb(1), and one Nb(4) atom. In the twelfth O site, O(12) is bonded to one Sr(3), one Sr(4), one Sr(6), one Nb(4), and one Nb(6) atom to form a mixture of distorted corner and edge-sharing OSr3Nb2 square pyramids. In the thirteenth O site, O(13) is bonded to one Sr(2), one Sr(4), one Sr(5), one Nb(4), and one Nb(5) atom to form a mixture of distorted corner and edge-sharing OSr3Nb2 square pyramids. In the fourteenth O site, O(14) is bonded in a distorted linear geometry to one Sr(2), one Sr(3), one Sr(4), one Sr(5), one Nb(2), and one Nb(5) atom. In the fifteenth O site, O(15) is bonded to one Sr(1), one Sr(4), one Sr(5), one Nb(5), and one Nb(7) atom to form a mixture of distorted face, corner, and edge-sharing OSr3Nb2 square pyramids. In the sixteenth O site, O(16) is bonded in a distorted linear geometry to one Sr(1), one Sr(3), one Sr(5), one Sr(6), one Nb(5), and one Nb(6) atom. In the seventeenth O site, O(17) is bonded in a distorted linear geometry to one Sr(1), one Sr(2), one Sr(5), one Sr(6), one Nb(1), and one Nb(6) atom. In the eighteenth O site, O(18) is bonded in a distorted linear geometry to one Sr(3), one Sr(4), one Sr(5), one Sr(6), one Nb(3), and one Nb(6) atom. In the nineteenth O site, O(19) is bonded to one Sr(2), one Sr(4), one Sr(6), one Nb(6), and one Nb(7) atom to form a mixture of distorted face, corner, and edge-sharing OSr3Nb2 square pyramids. In the twentieth O site, O(20) is bonded to one Sr(4), one Sr(5), one Sr(6), one Nb(4), and one Nb(7) atom to form a mixture of distorted face, corner, and edge-sharing OSr3Nb2 square pyramids. In the twenty-first O site, O(21) is bonded in a distorted linear geometry to one Sr(1), one Sr(5), one Sr(6), one Nb(1), and one Nb(5) atom. is represented by the CIF file [CIF]
data_Sr6Nb7O21
_symmetry_space_group_name_H-M   'P 1'
_cell_length_a   7.051
_cell_length_b   9.100
_cell_length_c   9.101
_cell_angle_alpha   113.566
_cell_angle_beta   104.963
_cell_angle_gamma   104.967
_symmetry_Int_Tables_number   1
_cell_volume   472.084
_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  0.563  0.280  0.421  1.0
  Sr  Sr1  1  0.142  0.572  0.860  1.0
  Sr  Sr2  1  0.713  0.855  0.284  1.0
  Sr  Sr3  1  0.287  0.144  0.718  1.0
  Sr  Sr4  1  0.851  0.422  0.140  1.0
  Sr  Sr5  1  0.428  0.716  0.573  1.0
  Nb  Nb6  1  0.069  0.284  0.426  1.0
  Nb  Nb7  1  0.644  0.576  0.858  1.0
  Nb  Nb8  1  0.210  0.859  0.283  1.0
  Nb  Nb9  1  0.786  0.142  0.717  1.0
  Nb  Nb10  1  0.353  0.425  0.142  1.0
  Nb  Nb11  1  0.930  0.718  0.575  1.0
  Nb  Nb12  1  0.499  1.000  1.000  1.0
  O  O13  1  0.558  0.780  0.927  1.0
  O  O14  1  0.779  0.147  0.221  1.0
  O  O15  1  0.143  0.073  0.369  1.0
  O  O16  1  0.357  0.429  0.643  1.0
  O  O17  1  0.712  0.358  0.788  1.0
  O  O18  1  0.778  0.634  0.706  1.0
  O  O19  1  0.925  0.708  0.073  1.0
  O  O20  1  0.285  0.644  0.213  1.0
  O  O21  1  0.363  0.929  0.147  1.0
  O  O22  1  0.500  0.001  0.499  1.0
  O  O23  1  0.929  0.221  0.576  1.0
  O  O24  1  0.854  0.928  0.631  1.0
  O  O25  1  0.071  0.293  0.928  1.0
  O  O26  1  0.496  0.500  0.998  1.0
  O  O27  1  0.440  0.221  0.072  1.0
  O  O28  1  0.644  0.573  0.355  1.0
  O  O29  1  0.000  0.501  0.498  1.0
  O  O30  1  0.072  0.785  0.428  1.0
  O  O31  1  0.219  0.854  0.779  1.0
  O  O32  1  0.633  0.072  0.853  1.0
  O  O33  1  0.222  0.365  0.294  1.0
[/CIF]
.