Ba3SrNb2O9 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 Nb(1)O6 octahedra. In the second Ba site, Ba(2) is bonded in a 9-coordinate 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 Nb(1)O6 octahedra and faces with two equivalent Ba(1)O12 cuboctahedra. The corner-sharing octahedral tilt angles are 15°. Nb(1) is bonded to three equivalent O(1) and three equivalent O(2) atoms to form distorted NbO6 octahedra that share corners with three equivalent Sr(1)O6 octahedra, faces with three equivalent Ba(1)O12 cuboctahedra, and  a faceface with one Nb(1)O6 octahedra. The corner-sharing octahedral tilt angles are 15°. 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 Nb(1) atoms to form a mixture of distorted corner and face-sharing OBa4Nb2 octahedra. The corner-sharing octahedral tilt angles range from 6-60°. 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 Nb(1) atom.
Ba3SrNb2O9 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 Nb(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 9-coordinate geometry to three equivalent O(1) and six equivalent O(2) atoms. All Ba(2)-O(1) bond lengths are 2.76 Å. All Ba(2)-O(2) bond lengths are 3.08 Å. Sr(1) is bonded to six equivalent O(2) atoms to form SrO6 octahedra that share corners with six equivalent Nb(1)O6 octahedra and faces with two equivalent Ba(1)O12 cuboctahedra. The corner-sharing octahedral tilt angles are 15°. All Sr(1)-O(2) bond lengths are 2.46 Å. Nb(1) is bonded to three equivalent O(1) and three equivalent O(2) atoms to form distorted NbO6 octahedra that share corners with three equivalent Sr(1)O6 octahedra, faces with three equivalent Ba(1)O12 cuboctahedra, and  a faceface with one Nb(1)O6 octahedra. The corner-sharing octahedral tilt angles are 15°. All Nb(1)-O(1) bond lengths are 2.20 Å. All Nb(1)-O(2) bond lengths are 1.91 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 Nb(1) atoms to form a mixture of distorted corner and face-sharing OBa4Nb2 octahedra. The corner-sharing octahedral tilt angles range from 6-60°. 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 Nb(1) atom.