Ba3SrTa2O9 crystallizes in the hexagonal P6_3/mmc space group. There are two inequivalent Ba sites. In the first Ba site, Ba(1) is bonded to six equivalent O(1) and six equivalent O(2) atoms to form BaO12 cuboctahedra that share corners with six equivalent Ba(1)O12 cuboctahedra, faces with two equivalent Sr(1)O6 octahedra, and faces with six equivalent Ta(1)O6 octahedra. In the second Ba site, Ba(2) is bonded in a distorted q6 geometry to three equivalent O(1) and six equivalent O(2) atoms. Sr(1) is bonded to six equivalent O(2) atoms to form SrO6 octahedra that share corners with six equivalent Ta(1)O6 octahedra and faces with two equivalent Ba(1)O12 cuboctahedra. The corner-sharing octahedral tilt angles are 14°. 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 Sr(1)O6 octahedra, faces with three equivalent Ba(1)O12 cuboctahedra, and  a faceface with one Ta(1)O6 octahedra. The corner-sharing octahedral tilt angles are 14°. There are two inequivalent O sites. In the first O site, O(1) is bonded to two equivalent Ba(1), two equivalent Ba(2), and two equivalent Ta(1) atoms to form a mixture of distorted face and corner-sharing OBa4Ta2 octahedra. The corner-sharing octahedral tilt angles range from 7-60°. In the second O site, O(2) is bonded in a 5-coordinate geometry to one Ba(1), two equivalent Ba(2), one Sr(1), and one Ta(1) atom.
Ba3SrTa2O9 crystallizes in the hexagonal P6_3/mmc space group. There are two inequivalent Ba sites. In the first Ba site, Ba(1) is bonded to six equivalent O(1) and six equivalent O(2) atoms to form BaO12 cuboctahedra that share corners with six equivalent Ba(1)O12 cuboctahedra, faces with two equivalent Sr(1)O6 octahedra, and faces with six equivalent Ta(1)O6 octahedra. All Ba(1)-O(1) bond lengths are 3.05 Å. All Ba(1)-O(2) bond lengths are 3.00 Å. In the second Ba site, Ba(2) is bonded in a distorted q6 geometry to three equivalent O(1) and six equivalent O(2) atoms. All Ba(2)-O(1) bond lengths are 2.81 Å. All Ba(2)-O(2) bond lengths are 3.07 Å. Sr(1) is bonded to six equivalent O(2) atoms to form SrO6 octahedra that share corners with six equivalent Ta(1)O6 octahedra and faces with two equivalent Ba(1)O12 cuboctahedra. The corner-sharing octahedral tilt angles are 14°. All Sr(1)-O(2) bond lengths are 2.45 Å. 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 Sr(1)O6 octahedra, faces with three equivalent Ba(1)O12 cuboctahedra, and  a faceface with one Ta(1)O6 octahedra. The corner-sharing octahedral tilt angles are 14°. All Ta(1)-O(1) bond lengths are 2.17 Å. All Ta(1)-O(2) bond lengths are 1.90 Å. There are two inequivalent O sites. In the first O site, O(1) is bonded to two equivalent Ba(1), two equivalent Ba(2), and two equivalent Ta(1) atoms to form a mixture of distorted face and corner-sharing OBa4Ta2 octahedra. The corner-sharing octahedral tilt angles range from 7-60°. In the second O site, O(2) is bonded in a 5-coordinate geometry to one Ba(1), two equivalent Ba(2), one Sr(1), and one Ta(1) atom.