The structure described by Sr4Nb2O9 crystallizes in the triclinic P1 space group. There are eight inequivalent Sr sites. In the first Sr site, Sr(1) is bonded in a 7-coordinate geometry to one O(1), one O(11), one O(16), one O(18), one O(3), one O(6), and one O(7) atom. In the second Sr site, Sr(2) is bonded to one O(1), one O(2), one O(3), one O(4), one O(6), one O(7), and one O(8) atom to form distorted SrO7 pentagonal bipyramids that share corners with three equivalent Nb(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 15-33°. In the third Sr site, Sr(3) is bonded in a 7-coordinate geometry to one O(10), one O(4), one O(5), one O(8), one O(9), and two equivalent O(13) atoms. In the fourth Sr site, Sr(4) is bonded in a 6-coordinate geometry to one O(11), one O(14), one O(15), one O(16), one O(17), and one O(18) atom. In the fifth Sr site, Sr(5) is bonded in a 6-coordinate geometry to one O(1), one O(2), one O(3), one O(4), one O(7), and one O(8) atom. In the sixth Sr site, Sr(6) is bonded in a 7-coordinate geometry to one O(10), one O(12), one O(13), one O(14), one O(17), one O(5), and one O(9) atom. In the seventh Sr site, Sr(7) is bonded to one O(11), one O(14), one O(15), one O(16), one O(17), and one O(18) atom to form distorted SrO6 pentagonal pyramids that share corners with three equivalent Nb(1)O6 octahedra and corners with three equivalent Nb(3)O5 trigonal bipyramids. The corner-sharing octahedral tilt angles range from 8-25°. In the eighth Sr site, Sr(8) is bonded in a 7-coordinate geometry to one O(1), one O(11), one O(15), one O(16), one O(18), one O(3), and one O(6) atom. There are four inequivalent Nb sites. In the first Nb site, Nb(1) is bonded to one O(1), one O(11), one O(16), one O(18), one O(3), and one O(6) atom to form NbO6 octahedra that share corners with three equivalent Sr(2)O7 pentagonal bipyramids and corners with three equivalent Sr(7)O6 pentagonal pyramids. In the second Nb site, Nb(2) is bonded in a 5-coordinate geometry to one O(13), one O(2), one O(4), one O(7), and one O(8) atom. In the third Nb site, Nb(3) is bonded to one O(12), one O(14), one O(15), one O(17), and one O(5) atom to form distorted NbO5 trigonal bipyramids that share corners with three equivalent Sr(7)O6 pentagonal pyramids and  a cornercorner with one Nb(4)O4 tetrahedra. In the fourth Nb site, Nb(4) is bonded to one O(10), one O(12), one O(13), and one O(9) atom to form corner-sharing NbO4 tetrahedra. There are eighteen inequivalent O sites. In the first O site, O(1) is bonded in a 5-coordinate geometry to one Sr(1), one Sr(2), one Sr(5), one Sr(8), and one Nb(1) atom. In the second O site, O(2) is bonded in a 3-coordinate geometry to one Sr(2), one Sr(5), and one Nb(2) atom. In the third O site, O(3) is bonded in a 5-coordinate geometry to one Sr(1), one Sr(2), one Sr(5), one Sr(8), and one Nb(1) atom. In the fourth O site, O(4) is bonded in a distorted see-saw-like geometry to one Sr(2), one Sr(3), one Sr(5), and one Nb(2) atom. In the fifth O site, O(5) is bonded in a distorted single-bond geometry to one Sr(3), one Sr(6), and one Nb(3) atom. In the sixth O site, O(6) is bonded in a 4-coordinate geometry to one Sr(1), one Sr(2), one Sr(8), and one Nb(1) atom. In the seventh O site, O(7) is bonded to one Sr(1), one Sr(2), one Sr(5), and one Nb(2) atom to form distorted OSr3Nb tetrahedra that share  a cornercorner with one O(8)Sr3Nb tetrahedra,  a cornercorner with one O(18)Sr4Nb trigonal bipyramid, and  an edgeedge with one O(8)Sr3Nb tetrahedra. In the eighth O site, O(8) is bonded to one Sr(2), one Sr(3), one Sr(5), and one Nb(2) atom to form a mixture of distorted corner and edge-sharing OSr3Nb tetrahedra. In the ninth O site, O(9) is bonded in a 3-coordinate geometry to one Sr(3), one Sr(6), and one Nb(4) atom. In the tenth O site, O(10) is bonded in a distorted trigonal planar geometry to one Sr(3), one Sr(6), and one Nb(4) atom. In the eleventh O site, O(11) is bonded in a 5-coordinate geometry to one Sr(1), one Sr(4), one Sr(7), one Sr(8), and one Nb(1) atom. In the twelfth O site, O(12) is bonded in a distorted T-shaped geometry to one Sr(6), one Nb(3), and one Nb(4) atom. In the thirteenth O site, O(13) is bonded in a 2-coordinate geometry to one Sr(6), two equivalent Sr(3), one Nb(2), and one Nb(4) atom. In the fourteenth O site, O(14) is bonded to one Sr(4), one Sr(6), one Sr(7), and one Nb(3) atom to form distorted corner-sharing OSr3Nb trigonal pyramids. In the fifteenth O site, O(15) is bonded in a distorted rectangular see-saw-like geometry to one Sr(4), one Sr(7), one Sr(8), and one Nb(3) atom. In the sixteenth O site, O(16) is bonded in a 5-coordinate geometry to one Sr(1), one Sr(4), one Sr(7), one Sr(8), and one Nb(1) atom. In the seventeenth O site, O(17) is bonded in a 4-coordinate geometry to one Sr(4), one Sr(6), one Sr(7), and one Nb(3) atom. In the eighteenth O site, O(18) is bonded to one Sr(1), one Sr(4), one Sr(7), one Sr(8), and one Nb(1) atom to form distorted corner-sharing OSr4Nb trigonal bipyramids. is represented by the Crystallographic Information File (CIF) [CIF]
data_Sr4Nb2O9
_symmetry_space_group_name_H-M   'P 1'
_cell_length_a   6.016
_cell_length_b   6.021
_cell_length_c   15.743
_cell_angle_alpha   94.034
_cell_angle_beta   94.111
_cell_angle_gamma   118.499
_symmetry_Int_Tables_number   1
_cell_volume   496.304
_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
  Sr  Sr0  1  0.336  0.682  0.951  1.0
  Sr  Sr1  1  0.621  0.302  0.817  1.0
  Sr  Sr2  1  0.718  0.432  0.568  1.0
  Sr  Sr3  1  0.075  0.030  0.203  1.0
  Sr  Sr4  1  0.955  0.975  0.775  1.0
  Sr  Sr5  1  0.441  0.761  0.410  1.0
  Sr  Sr6  1  0.396  0.688  0.168  1.0
  Sr  Sr7  1  0.692  0.343  0.048  1.0
  Nb  Nb8  1  0.009  0.986  0.992  1.0
  Nb  Nb9  1  0.266  0.557  0.704  1.0
  Nb  Nb10  1  0.778  0.377  0.308  1.0
  Nb  Nb11  1  0.052  0.105  0.498  1.0
  O  O12  1  0.784  0.044  0.908  1.0
  O  O13  1  0.290  0.878  0.726  1.0
  O  O14  1  0.307  0.111  0.927  1.0
  O  O15  1  0.908  0.307  0.694  1.0
  O  O16  1  0.779  0.568  0.406  1.0
  O  O17  1  0.888  0.629  0.935  1.0
  O  O18  1  0.289  0.449  0.814  1.0
  O  O19  1  0.596  0.575  0.695  1.0
  O  O20  1  0.786  0.872  0.549  1.0
  O  O21  1  0.333  0.049  0.490  1.0
  O  O22  1  0.117  0.323  0.066  1.0
  O  O23  1  0.883  0.131  0.397  1.0
  O  O24  1  0.194  0.435  0.562  1.0
  O  O25  1  0.112  0.446  0.280  1.0
  O  O26  1  0.714  0.548  0.220  1.0
  O  O27  1  0.725  0.823  0.071  1.0
  O  O28  1  0.480  0.047  0.282  1.0
  O  O29  1  0.252  0.931  0.084  1.0
[/CIF]
.
The structure described by Sr4Nb2O9 crystallizes in the triclinic P1 space group. There are eight inequivalent Sr sites. In the first Sr site, Sr(1) is bonded in a 7-coordinate geometry to one O(1), one O(11), one O(16), one O(18), one O(3), one O(6), and one O(7) atom. In the second Sr site, Sr(2) is bonded to one O(1), one O(2), one O(3), one O(4), one O(6), one O(7), and one O(8) atom to form distorted SrO7 pentagonal bipyramids that share corners with three equivalent Nb(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 15-33°. In the third Sr site, Sr(3) is bonded in a 7-coordinate geometry to one O(10), one O(4), one O(5), one O(8), one O(9), and two equivalent O(13) atoms. In the fourth Sr site, Sr(4) is bonded in a 6-coordinate geometry to one O(11), one O(14), one O(15), one O(16), one O(17), and one O(18) atom. In the fifth Sr site, Sr(5) is bonded in a 6-coordinate geometry to one O(1), one O(2), one O(3), one O(4), one O(7), and one O(8) atom. In the sixth Sr site, Sr(6) is bonded in a 7-coordinate geometry to one O(10), one O(12), one O(13), one O(14), one O(17), one O(5), and one O(9) atom. In the seventh Sr site, Sr(7) is bonded to one O(11), one O(14), one O(15), one O(16), one O(17), and one O(18) atom to form distorted SrO6 pentagonal pyramids that share corners with three equivalent Nb(1)O6 octahedra and corners with three equivalent Nb(3)O5 trigonal bipyramids. The corner-sharing octahedral tilt angles range from 8-25°. In the eighth Sr site, Sr(8) is bonded in a 7-coordinate geometry to one O(1), one O(11), one O(15), one O(16), one O(18), one O(3), and one O(6) atom. There are four inequivalent Nb sites. In the first Nb site, Nb(1) is bonded to one O(1), one O(11), one O(16), one O(18), one O(3), and one O(6) atom to form NbO6 octahedra that share corners with three equivalent Sr(2)O7 pentagonal bipyramids and corners with three equivalent Sr(7)O6 pentagonal pyramids. In the second Nb site, Nb(2) is bonded in a 5-coordinate geometry to one O(13), one O(2), one O(4), one O(7), and one O(8) atom. In the third Nb site, Nb(3) is bonded to one O(12), one O(14), one O(15), one O(17), and one O(5) atom to form distorted NbO5 trigonal bipyramids that share corners with three equivalent Sr(7)O6 pentagonal pyramids and  a cornercorner with one Nb(4)O4 tetrahedra. In the fourth Nb site, Nb(4) is bonded to one O(10), one O(12), one O(13), and one O(9) atom to form corner-sharing NbO4 tetrahedra. There are eighteen inequivalent O sites. In the first O site, O(1) is bonded in a 5-coordinate geometry to one Sr(1), one Sr(2), one Sr(5), one Sr(8), and one Nb(1) atom. In the second O site, O(2) is bonded in a 3-coordinate geometry to one Sr(2), one Sr(5), and one Nb(2) atom. In the third O site, O(3) is bonded in a 5-coordinate geometry to one Sr(1), one Sr(2), one Sr(5), one Sr(8), and one Nb(1) atom. In the fourth O site, O(4) is bonded in a distorted see-saw-like geometry to one Sr(2), one Sr(3), one Sr(5), and one Nb(2) atom. In the fifth O site, O(5) is bonded in a distorted single-bond geometry to one Sr(3), one Sr(6), and one Nb(3) atom. In the sixth O site, O(6) is bonded in a 4-coordinate geometry to one Sr(1), one Sr(2), one Sr(8), and one Nb(1) atom. In the seventh O site, O(7) is bonded to one Sr(1), one Sr(2), one Sr(5), and one Nb(2) atom to form distorted OSr3Nb tetrahedra that share  a cornercorner with one O(8)Sr3Nb tetrahedra,  a cornercorner with one O(18)Sr4Nb trigonal bipyramid, and  an edgeedge with one O(8)Sr3Nb tetrahedra. In the eighth O site, O(8) is bonded to one Sr(2), one Sr(3), one Sr(5), and one Nb(2) atom to form a mixture of distorted corner and edge-sharing OSr3Nb tetrahedra. In the ninth O site, O(9) is bonded in a 3-coordinate geometry to one Sr(3), one Sr(6), and one Nb(4) atom. In the tenth O site, O(10) is bonded in a distorted trigonal planar geometry to one Sr(3), one Sr(6), and one Nb(4) atom. In the eleventh O site, O(11) is bonded in a 5-coordinate geometry to one Sr(1), one Sr(4), one Sr(7), one Sr(8), and one Nb(1) atom. In the twelfth O site, O(12) is bonded in a distorted T-shaped geometry to one Sr(6), one Nb(3), and one Nb(4) atom. In the thirteenth O site, O(13) is bonded in a 2-coordinate geometry to one Sr(6), two equivalent Sr(3), one Nb(2), and one Nb(4) atom. In the fourteenth O site, O(14) is bonded to one Sr(4), one Sr(6), one Sr(7), and one Nb(3) atom to form distorted corner-sharing OSr3Nb trigonal pyramids. In the fifteenth O site, O(15) is bonded in a distorted rectangular see-saw-like geometry to one Sr(4), one Sr(7), one Sr(8), and one Nb(3) atom. In the sixteenth O site, O(16) is bonded in a 5-coordinate geometry to one Sr(1), one Sr(4), one Sr(7), one Sr(8), and one Nb(1) atom. In the seventeenth O site, O(17) is bonded in a 4-coordinate geometry to one Sr(4), one Sr(6), one Sr(7), and one Nb(3) atom. In the eighteenth O site, O(18) is bonded to one Sr(1), one Sr(4), one Sr(7), one Sr(8), and one Nb(1) atom to form distorted corner-sharing OSr4Nb trigonal bipyramids. is represented by the Crystallographic Information File (CIF) [CIF]
data_Sr4Nb2O9
_symmetry_space_group_name_H-M   'P 1'
_cell_length_a   6.016
_cell_length_b   6.021
_cell_length_c   15.743
_cell_angle_alpha   94.034
_cell_angle_beta   94.111
_cell_angle_gamma   118.499
_symmetry_Int_Tables_number   1
_cell_volume   496.304
_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
  Sr  Sr0  1  0.336  0.682  0.951  1.0
  Sr  Sr1  1  0.621  0.302  0.817  1.0
  Sr  Sr2  1  0.718  0.432  0.568  1.0
  Sr  Sr3  1  0.075  0.030  0.203  1.0
  Sr  Sr4  1  0.955  0.975  0.775  1.0
  Sr  Sr5  1  0.441  0.761  0.410  1.0
  Sr  Sr6  1  0.396  0.688  0.168  1.0
  Sr  Sr7  1  0.692  0.343  0.048  1.0
  Nb  Nb8  1  0.009  0.986  0.992  1.0
  Nb  Nb9  1  0.266  0.557  0.704  1.0
  Nb  Nb10  1  0.778  0.377  0.308  1.0
  Nb  Nb11  1  0.052  0.105  0.498  1.0
  O  O12  1  0.784  0.044  0.908  1.0
  O  O13  1  0.290  0.878  0.726  1.0
  O  O14  1  0.307  0.111  0.927  1.0
  O  O15  1  0.908  0.307  0.694  1.0
  O  O16  1  0.779  0.568  0.406  1.0
  O  O17  1  0.888  0.629  0.935  1.0
  O  O18  1  0.289  0.449  0.814  1.0
  O  O19  1  0.596  0.575  0.695  1.0
  O  O20  1  0.786  0.872  0.549  1.0
  O  O21  1  0.333  0.049  0.490  1.0
  O  O22  1  0.117  0.323  0.066  1.0
  O  O23  1  0.883  0.131  0.397  1.0
  O  O24  1  0.194  0.435  0.562  1.0
  O  O25  1  0.112  0.446  0.280  1.0
  O  O26  1  0.714  0.548  0.220  1.0
  O  O27  1  0.725  0.823  0.071  1.0
  O  O28  1  0.480  0.047  0.282  1.0
  O  O29  1  0.252  0.931  0.084  1.0
[/CIF]
.