The compound described by Ba4Ta2O9 is Krennerite-derived structured and crystallizes in the hexagonal P6_3/m space group. There are three inequivalent Ba sites. In the first Ba site, Ba(1) is bonded in a 9-coordinate geometry to three equivalent O(1) and six equivalent O(2) atoms. In the second Ba site, Ba(2) is bonded in a 6-coordinate geometry to three equivalent O(1) and three equivalent O(2) atoms. In the third Ba site, Ba(3) is bonded to six equivalent O(2) atoms to form distorted BaO6 octahedra that share corners with six equivalent Ta(1)O6 octahedra. The corner-sharing octahedral tilt angles are 27°. Ta(1) is bonded to three equivalent O(1) and three equivalent O(2) atoms to form distorted TaO6 octahedra that share corners with three equivalent Ba(3)O6 octahedra and  a faceface with one Ta(1)O6 octahedra. The corner-sharing octahedral tilt angles are 27°. There are two inequivalent O sites. In the first O site, O(1) is bonded in a 5-coordinate geometry to one Ba(1), two equivalent Ba(2), and two equivalent Ta(1) atoms. In the second O site, O(2) is bonded in a 1-coordinate geometry to one Ba(1), one Ba(2), one Ba(3), and one Ta(1) atom. is represented by the CIF card [CIF]
data_Ba4Ta2O9
_symmetry_space_group_name_H-M   'P 1'
_cell_length_a   6.124
_cell_length_b   6.124
_cell_length_c   16.617
_cell_angle_alpha   90.000
_cell_angle_beta   90.000
_cell_angle_gamma   120.000
_symmetry_Int_Tables_number   1
_cell_volume   539.630
_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
  Ba  Ba0  1  1.000  1.000  0.750  1.0
  Ba  Ba1  1  0.000  0.000  0.250  1.0
  Ba  Ba2  1  0.667  0.333  0.634  1.0
  Ba  Ba3  1  0.333  0.667  0.134  1.0
  Ba  Ba4  1  0.333  0.667  0.366  1.0
  Ba  Ba5  1  0.667  0.333  0.866  1.0
  Ba  Ba6  1  1.000  1.000  0.500  1.0
  Ba  Ba7  1  1.000  0.000  1.000  1.0
  Ta  Ta8  1  0.667  0.333  0.340  1.0
  Ta  Ta9  1  0.333  0.667  0.840  1.0
  Ta  Ta10  1  0.333  0.667  0.660  1.0
  Ta  Ta11  1  0.667  0.333  0.160  1.0
  O  O12  1  0.525  0.953  0.750  1.0
  O  O13  1  0.047  0.572  0.750  1.0
  O  O14  1  0.428  0.475  0.750  1.0
  O  O15  1  0.475  0.047  0.250  1.0
  O  O16  1  0.953  0.428  0.250  1.0
  O  O17  1  0.572  0.525  0.250  1.0
  O  O18  1  0.111  0.357  0.609  1.0
  O  O19  1  0.643  0.754  0.609  1.0
  O  O20  1  0.246  0.889  0.609  1.0
  O  O21  1  0.889  0.643  0.109  1.0
  O  O22  1  0.357  0.246  0.109  1.0
  O  O23  1  0.754  0.111  0.109  1.0
  O  O24  1  0.889  0.643  0.391  1.0
  O  O25  1  0.357  0.246  0.391  1.0
  O  O26  1  0.754  0.111  0.391  1.0
  O  O27  1  0.111  0.357  0.891  1.0
  O  O28  1  0.643  0.754  0.891  1.0
  O  O29  1  0.246  0.889  0.891  1.0
[/CIF]
.
The compound described by Ba4Ta2O9 is Krennerite-derived structured and crystallizes in the hexagonal P6_3/m space group. There are three inequivalent Ba sites. In the first Ba site, Ba(1) is bonded in a 9-coordinate geometry to three equivalent O(1) and six equivalent O(2) atoms. In the second Ba site, Ba(2) is bonded in a 6-coordinate geometry to three equivalent O(1) and three equivalent O(2) atoms. In the third Ba site, Ba(3) is bonded to six equivalent O(2) atoms to form distorted BaO6 octahedra that share corners with six equivalent Ta(1)O6 octahedra. The corner-sharing octahedral tilt angles are 27°. Ta(1) is bonded to three equivalent O(1) and three equivalent O(2) atoms to form distorted TaO6 octahedra that share corners with three equivalent Ba(3)O6 octahedra and  a faceface with one Ta(1)O6 octahedra. The corner-sharing octahedral tilt angles are 27°. There are two inequivalent O sites. In the first O site, O(1) is bonded in a 5-coordinate geometry to one Ba(1), two equivalent Ba(2), and two equivalent Ta(1) atoms. In the second O site, O(2) is bonded in a 1-coordinate geometry to one Ba(1), one Ba(2), one Ba(3), and one Ta(1) atom. is represented by the CIF card [CIF]
data_Ba4Ta2O9
_symmetry_space_group_name_H-M   'P 1'
_cell_length_a   6.124
_cell_length_b   6.124
_cell_length_c   16.617
_cell_angle_alpha   90.000
_cell_angle_beta   90.000
_cell_angle_gamma   120.000
_symmetry_Int_Tables_number   1
_cell_volume   539.630
_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
  Ba  Ba0  1  1.000  1.000  0.750  1.0
  Ba  Ba1  1  0.000  0.000  0.250  1.0
  Ba  Ba2  1  0.667  0.333  0.634  1.0
  Ba  Ba3  1  0.333  0.667  0.134  1.0
  Ba  Ba4  1  0.333  0.667  0.366  1.0
  Ba  Ba5  1  0.667  0.333  0.866  1.0
  Ba  Ba6  1  1.000  1.000  0.500  1.0
  Ba  Ba7  1  1.000  0.000  1.000  1.0
  Ta  Ta8  1  0.667  0.333  0.340  1.0
  Ta  Ta9  1  0.333  0.667  0.840  1.0
  Ta  Ta10  1  0.333  0.667  0.660  1.0
  Ta  Ta11  1  0.667  0.333  0.160  1.0
  O  O12  1  0.525  0.953  0.750  1.0
  O  O13  1  0.047  0.572  0.750  1.0
  O  O14  1  0.428  0.475  0.750  1.0
  O  O15  1  0.475  0.047  0.250  1.0
  O  O16  1  0.953  0.428  0.250  1.0
  O  O17  1  0.572  0.525  0.250  1.0
  O  O18  1  0.111  0.357  0.609  1.0
  O  O19  1  0.643  0.754  0.609  1.0
  O  O20  1  0.246  0.889  0.609  1.0
  O  O21  1  0.889  0.643  0.109  1.0
  O  O22  1  0.357  0.246  0.109  1.0
  O  O23  1  0.754  0.111  0.109  1.0
  O  O24  1  0.889  0.643  0.391  1.0
  O  O25  1  0.357  0.246  0.391  1.0
  O  O26  1  0.754  0.111  0.391  1.0
  O  O27  1  0.111  0.357  0.891  1.0
  O  O28  1  0.643  0.754  0.891  1.0
  O  O29  1  0.246  0.889  0.891  1.0
[/CIF]
.