The structure described by Cr8O21 crystallizes in the triclinic P-1 space group. There are four inequivalent Cr sites. In the first Cr site, Cr(1) is bonded to one O(10), one O(3), one O(5), and one O(8) atom to form corner-sharing CrO4 tetrahedra. In the second Cr site, Cr(2) is bonded to one O(11), one O(4), one O(6), one O(7), and two equivalent O(1) atoms to form CrO6 octahedra that share corners with two equivalent Cr(3)O4 tetrahedra, corners with four equivalent Cr(4)O4 tetrahedra, and  an edgeedge with one Cr(2)O6 octahedra. In the third Cr site, Cr(3) is bonded to one O(3), one O(6), one O(7), and one O(9) atom to form CrO4 tetrahedra that share corners with two equivalent Cr(2)O6 octahedra and  a cornercorner with one Cr(1)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 18-39°. In the fourth Cr site, Cr(4) is bonded to one O(1), one O(11), one O(2), and one O(4) atom to form corner-sharing CrO4 tetrahedra. The corner-sharing octahedral tilt angles range from 21-58°. There are eleven inequivalent O sites. In the first O site, O(1) is bonded in a trigonal planar geometry to one Cr(4) and two equivalent Cr(2) atoms. In the second O site, O(2) is bonded in a distorted water-like geometry to one Cr(4) and one O(9) atom. In the third O site, O(3) is bonded in a bent 150 degrees geometry to one Cr(1) and one Cr(3) atom. In the fourth O site, O(4) is bonded in a bent 150 degrees geometry to one Cr(2) and one Cr(4) atom. In the fifth O site, O(5) is bonded in a single-bond geometry to one Cr(1) atom. In the sixth O site, O(6) is bonded in a bent 150 degrees geometry to one Cr(2) and one Cr(3) atom. In the seventh O site, O(7) is bonded in a linear geometry to one Cr(2) and one Cr(3) atom. In the eighth O site, O(8) is bonded in a single-bond geometry to one Cr(1) atom. In the ninth O site, O(9) is bonded in a distorted single-bond geometry to one Cr(3) and one O(2) atom. In the tenth O site, O(10) is bonded in a linear geometry to two equivalent Cr(1) atoms. In the eleventh O site, O(11) is bonded in a bent 150 degrees geometry to one Cr(2) and one Cr(4) atom. is represented by the CIF card [CIF]
data_Cr8O21
_symmetry_space_group_name_H-M   'P 1'
_cell_length_a   5.428
_cell_length_b   6.593
_cell_length_c   12.691
_cell_angle_alpha   100.361
_cell_angle_beta   89.503
_cell_angle_gamma   97.355
_symmetry_Int_Tables_number   1
_cell_volume   443.019
_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
  Cr  Cr0  1  0.269  0.802  0.529  1.0
  Cr  Cr1  1  0.137  0.205  0.070  1.0
  Cr  Cr2  1  0.366  0.626  0.755  1.0
  Cr  Cr3  1  0.666  0.252  0.905  1.0
  Cr  Cr4  1  0.334  0.748  0.095  1.0
  Cr  Cr5  1  0.863  0.795  0.930  1.0
  Cr  Cr6  1  0.634  0.374  0.245  1.0
  Cr  Cr7  1  0.731  0.198  0.471  1.0
  O  O8  1  0.098  0.896  0.063  1.0
  O  O9  1  0.449  0.762  0.233  1.0
  O  O10  1  0.626  0.348  0.383  1.0
  O  O11  1  0.197  0.503  0.067  1.0
  O  O12  1  0.963  0.099  0.416  1.0
  O  O13  1  0.640  0.761  0.807  1.0
  O  O14  1  0.140  0.738  0.825  1.0
  O  O15  1  0.822  0.354  0.579  1.0
  O  O16  1  0.860  0.262  0.175  1.0
  O  O17  1  0.352  0.359  0.774  1.0
  O  O18  1  0.374  0.652  0.617  1.0
  O  O19  1  0.500  0.000  0.500  1.0
  O  O20  1  0.591  0.828  0.032  1.0
  O  O21  1  0.360  0.239  0.193  1.0
  O  O22  1  0.178  0.646  0.421  1.0
  O  O23  1  0.648  0.641  0.226  1.0
  O  O24  1  0.803  0.497  0.933  1.0
  O  O25  1  0.037  0.901  0.584  1.0
  O  O26  1  0.551  0.238  0.767  1.0
  O  O27  1  0.409  0.172  0.968  1.0
  O  O28  1  0.902  0.104  0.937  1.0
[/CIF]
.
The structure described by Cr8O21 crystallizes in the triclinic P-1 space group. There are four inequivalent Cr sites. In the first Cr site, Cr(1) is bonded to one O(10), one O(3), one O(5), and one O(8) atom to form corner-sharing CrO4 tetrahedra. In the second Cr site, Cr(2) is bonded to one O(11), one O(4), one O(6), one O(7), and two equivalent O(1) atoms to form CrO6 octahedra that share corners with two equivalent Cr(3)O4 tetrahedra, corners with four equivalent Cr(4)O4 tetrahedra, and  an edgeedge with one Cr(2)O6 octahedra. In the third Cr site, Cr(3) is bonded to one O(3), one O(6), one O(7), and one O(9) atom to form CrO4 tetrahedra that share corners with two equivalent Cr(2)O6 octahedra and  a cornercorner with one Cr(1)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 18-39°. In the fourth Cr site, Cr(4) is bonded to one O(1), one O(11), one O(2), and one O(4) atom to form corner-sharing CrO4 tetrahedra. The corner-sharing octahedral tilt angles range from 21-58°. There are eleven inequivalent O sites. In the first O site, O(1) is bonded in a trigonal planar geometry to one Cr(4) and two equivalent Cr(2) atoms. In the second O site, O(2) is bonded in a distorted water-like geometry to one Cr(4) and one O(9) atom. In the third O site, O(3) is bonded in a bent 150 degrees geometry to one Cr(1) and one Cr(3) atom. In the fourth O site, O(4) is bonded in a bent 150 degrees geometry to one Cr(2) and one Cr(4) atom. In the fifth O site, O(5) is bonded in a single-bond geometry to one Cr(1) atom. In the sixth O site, O(6) is bonded in a bent 150 degrees geometry to one Cr(2) and one Cr(3) atom. In the seventh O site, O(7) is bonded in a linear geometry to one Cr(2) and one Cr(3) atom. In the eighth O site, O(8) is bonded in a single-bond geometry to one Cr(1) atom. In the ninth O site, O(9) is bonded in a distorted single-bond geometry to one Cr(3) and one O(2) atom. In the tenth O site, O(10) is bonded in a linear geometry to two equivalent Cr(1) atoms. In the eleventh O site, O(11) is bonded in a bent 150 degrees geometry to one Cr(2) and one Cr(4) atom. is represented by the CIF card [CIF]
data_Cr8O21
_symmetry_space_group_name_H-M   'P 1'
_cell_length_a   5.428
_cell_length_b   6.593
_cell_length_c   12.691
_cell_angle_alpha   100.361
_cell_angle_beta   89.503
_cell_angle_gamma   97.355
_symmetry_Int_Tables_number   1
_cell_volume   443.019
_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
  Cr  Cr0  1  0.269  0.802  0.529  1.0
  Cr  Cr1  1  0.137  0.205  0.070  1.0
  Cr  Cr2  1  0.366  0.626  0.755  1.0
  Cr  Cr3  1  0.666  0.252  0.905  1.0
  Cr  Cr4  1  0.334  0.748  0.095  1.0
  Cr  Cr5  1  0.863  0.795  0.930  1.0
  Cr  Cr6  1  0.634  0.374  0.245  1.0
  Cr  Cr7  1  0.731  0.198  0.471  1.0
  O  O8  1  0.098  0.896  0.063  1.0
  O  O9  1  0.449  0.762  0.233  1.0
  O  O10  1  0.626  0.348  0.383  1.0
  O  O11  1  0.197  0.503  0.067  1.0
  O  O12  1  0.963  0.099  0.416  1.0
  O  O13  1  0.640  0.761  0.807  1.0
  O  O14  1  0.140  0.738  0.825  1.0
  O  O15  1  0.822  0.354  0.579  1.0
  O  O16  1  0.860  0.262  0.175  1.0
  O  O17  1  0.352  0.359  0.774  1.0
  O  O18  1  0.374  0.652  0.617  1.0
  O  O19  1  0.500  0.000  0.500  1.0
  O  O20  1  0.591  0.828  0.032  1.0
  O  O21  1  0.360  0.239  0.193  1.0
  O  O22  1  0.178  0.646  0.421  1.0
  O  O23  1  0.648  0.641  0.226  1.0
  O  O24  1  0.803  0.497  0.933  1.0
  O  O25  1  0.037  0.901  0.584  1.0
  O  O26  1  0.551  0.238  0.767  1.0
  O  O27  1  0.409  0.172  0.968  1.0
  O  O28  1  0.902  0.104  0.937  1.0
[/CIF]
.