Ba5Nb2O9(O2) crystallizes in the monoclinic C2/c space group. There are three inequivalent Ba sites. In the first Ba site, Ba(1) is bonded to two equivalent O(1), two equivalent O(2), two equivalent O(3), two equivalent O(5), and four equivalent O(4) atoms to form BaO12 cuboctahedra that share corners with six equivalent Ba(1)O12 cuboctahedra and faces with six equivalent Nb(1)O6 octahedra. In the second Ba site, Ba(2) is bonded in a 9-coordinate geometry to two equivalent O(1), two equivalent O(2), two equivalent O(3), and three equivalent O(6) atoms. In the third Ba site, Ba(3) is bonded in a 11-coordinate geometry to one O(5), two equivalent O(1), two equivalent O(2), two equivalent O(3), two equivalent O(4), and two equivalent O(6) atoms. Nb(1) is bonded to one O(1), one O(2), one O(3), one O(5), and two equivalent O(4) atoms to form NbO6 octahedra that share faces with three equivalent Ba(1)O12 cuboctahedra and  a faceface with one Nb(1)O6 octahedra. There are six inequivalent O sites. In the first O site, O(4) is bonded to two equivalent Ba(1), two equivalent Ba(3), 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 8-60°. In the second O site, O(5) is bonded to two equivalent Ba(1), two equivalent Ba(3), 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 7-60°. In the third O site, O(6) is bonded in a 6-coordinate geometry to two equivalent Ba(3), three equivalent Ba(2), and one O(6) atom. In the fourth O site, O(1) is bonded in a 6-coordinate geometry to one Ba(1), two equivalent Ba(2), two equivalent Ba(3), and one Nb(1) atom. In the fifth O site, O(2) is bonded in a 6-coordinate geometry to one Ba(1), two equivalent Ba(2), two equivalent Ba(3), and one Nb(1) atom. In the sixth O site, O(3) is bonded in a 6-coordinate geometry to one Ba(1), two equivalent Ba(2), two equivalent Ba(3), and one Nb(1) atom.
Ba5Nb2O9(O2) crystallizes in the monoclinic C2/c space group. There are three inequivalent Ba sites. In the first Ba site, Ba(1) is bonded to two equivalent O(1), two equivalent O(2), two equivalent O(3), two equivalent O(5), and four equivalent O(4) atoms to form BaO12 cuboctahedra that share corners with six equivalent Ba(1)O12 cuboctahedra and faces with six equivalent Nb(1)O6 octahedra. Both Ba(1)-O(1) bond lengths are 3.03 Å. Both Ba(1)-O(2) bond lengths are 3.12 Å. Both Ba(1)-O(3) bond lengths are 3.04 Å. Both Ba(1)-O(5) bond lengths are 3.08 Å. There are two shorter (3.07 Å) and two longer (3.08 Å) Ba(1)-O(4) bond lengths. In the second Ba site, Ba(2) is bonded in a 9-coordinate geometry to two equivalent O(1), two equivalent O(2), two equivalent O(3), and three equivalent O(6) atoms. There is one shorter (2.60 Å) and one longer (3.12 Å) Ba(2)-O(1) bond length. There is one shorter (2.60 Å) and one longer (3.13 Å) Ba(2)-O(2) bond length. There is one shorter (2.59 Å) and one longer (3.13 Å) Ba(2)-O(3) bond length. There are a spread of Ba(2)-O(6) bond distances ranging from 2.76-3.28 Å. In the third Ba site, Ba(3) is bonded in a 11-coordinate geometry to one O(5), two equivalent O(1), two equivalent O(2), two equivalent O(3), two equivalent O(4), and two equivalent O(6) atoms. The Ba(3)-O(5) bond length is 2.84 Å. There is one shorter (3.08 Å) and one longer (3.10 Å) Ba(3)-O(1) bond length. Both Ba(3)-O(2) bond lengths are 3.10 Å. There is one shorter (3.08 Å) and one longer (3.10 Å) Ba(3)-O(3) bond length. There is one shorter (2.83 Å) and one longer (2.88 Å) Ba(3)-O(4) bond length. There is one shorter (2.68 Å) and one longer (2.74 Å) Ba(3)-O(6) bond length. Nb(1) is bonded to one O(1), one O(2), one O(3), one O(5), and two equivalent O(4) atoms to form NbO6 octahedra that share faces with three equivalent Ba(1)O12 cuboctahedra and  a faceface with one Nb(1)O6 octahedra. The Nb(1)-O(1) bond length is 1.92 Å. The Nb(1)-O(2) bond length is 1.94 Å. The Nb(1)-O(3) bond length is 1.92 Å. The Nb(1)-O(5) bond length is 2.16 Å. There is one shorter (2.15 Å) and one longer (2.16 Å) Nb(1)-O(4) bond length. There are six inequivalent O sites. In the first O site, O(4) is bonded to two equivalent Ba(1), two equivalent Ba(3), 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 8-60°. In the second O site, O(5) is bonded to two equivalent Ba(1), two equivalent Ba(3), 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 7-60°. In the third O site, O(6) is bonded in a 6-coordinate geometry to two equivalent Ba(3), three equivalent Ba(2), and one O(6) atom. The O(6)-O(6) bond length is 1.52 Å. In the fourth O site, O(1) is bonded in a 6-coordinate geometry to one Ba(1), two equivalent Ba(2), two equivalent Ba(3), and one Nb(1) atom. In the fifth O site, O(2) is bonded in a 6-coordinate geometry to one Ba(1), two equivalent Ba(2), two equivalent Ba(3), and one Nb(1) atom. In the sixth O site, O(3) is bonded in a 6-coordinate geometry to one Ba(1), two equivalent Ba(2), two equivalent Ba(3), and one Nb(1) atom.