Ba5Nb4O15 crystallizes in the trigonal P-3m1 space group. There are three inequivalent Ba sites. In the first Ba site, Ba(1) is bonded in a 12-coordinate geometry to three equivalent O(2) and nine equivalent O(1) atoms. In the second Ba site, Ba(2) is bonded to three equivalent O(1), three equivalent O(3), and six equivalent O(2) atoms to form BaO12 cuboctahedra that share corners with nine equivalent Ba(2)O12 cuboctahedra, faces with three equivalent Ba(3)O12 cuboctahedra, faces with three equivalent Nb(2)O6 octahedra, and faces with four equivalent Nb(1)O6 octahedra. In the third Ba site, Ba(3) is bonded to six equivalent O(2) and six equivalent O(3) atoms to form BaO12 cuboctahedra that share corners with six equivalent Ba(3)O12 cuboctahedra, faces with six equivalent Ba(2)O12 cuboctahedra, faces with two equivalent Nb(2)O6 octahedra, and faces with six equivalent Nb(1)O6 octahedra. There are two inequivalent Nb sites. In the first Nb site, Nb(1) is bonded to three equivalent O(2) and three equivalent O(3) atoms to form NbO6 octahedra that share corners with three equivalent Nb(1)O6 octahedra, corners with three equivalent Nb(2)O6 octahedra, faces with three equivalent Ba(3)O12 cuboctahedra, and faces with four equivalent Ba(2)O12 cuboctahedra. The corner-sharing octahedral tilt angles range from 0-9°. In the second Nb site, Nb(2) is bonded to three equivalent O(1) and three equivalent O(2) atoms to form distorted NbO6 octahedra that share corners with three equivalent Nb(1)O6 octahedra,  a faceface with one Ba(3)O12 cuboctahedra, and faces with three equivalent Ba(2)O12 cuboctahedra. The corner-sharing octahedral tilt angles are 9°. There are three inequivalent O sites. In the first O site, O(1) is bonded in a distorted single-bond geometry to one Ba(2), three equivalent Ba(1), and one Nb(2) atom. In the second O site, O(2) is bonded in a 2-coordinate geometry to one Ba(1), one Ba(3), two equivalent Ba(2), one Nb(1), and one Nb(2) atom. In the third O site, O(3) is bonded in a distorted linear geometry to two equivalent Ba(2), two equivalent Ba(3), and two equivalent Nb(1) atoms.
Ba5Nb4O15 crystallizes in the trigonal P-3m1 space group. There are three inequivalent Ba sites. In the first Ba site, Ba(1) is bonded in a 12-coordinate geometry to three equivalent O(2) and nine equivalent O(1) atoms. All Ba(1)-O(2) bond lengths are 3.28 Å. There are three shorter (2.78 Å) and six longer (2.97 Å) Ba(1)-O(1) bond lengths. In the second Ba site, Ba(2) is bonded to three equivalent O(1), three equivalent O(3), and six equivalent O(2) atoms to form BaO12 cuboctahedra that share corners with nine equivalent Ba(2)O12 cuboctahedra, faces with three equivalent Ba(3)O12 cuboctahedra, faces with three equivalent Nb(2)O6 octahedra, and faces with four equivalent Nb(1)O6 octahedra. All Ba(2)-O(1) bond lengths are 2.74 Å. All Ba(2)-O(3) bond lengths are 3.01 Å. All Ba(2)-O(2) bond lengths are 2.93 Å. In the third Ba site, Ba(3) is bonded to six equivalent O(2) and six equivalent O(3) atoms to form BaO12 cuboctahedra that share corners with six equivalent Ba(3)O12 cuboctahedra, faces with six equivalent Ba(2)O12 cuboctahedra, faces with two equivalent Nb(2)O6 octahedra, and faces with six equivalent Nb(1)O6 octahedra. All Ba(3)-O(2) bond lengths are 2.86 Å. All Ba(3)-O(3) bond lengths are 2.93 Å. There are two inequivalent Nb sites. In the first Nb site, Nb(1) is bonded to three equivalent O(2) and three equivalent O(3) atoms to form NbO6 octahedra that share corners with three equivalent Nb(1)O6 octahedra, corners with three equivalent Nb(2)O6 octahedra, faces with three equivalent Ba(3)O12 cuboctahedra, and faces with four equivalent Ba(2)O12 cuboctahedra. The corner-sharing octahedral tilt angles range from 0-9°. All Nb(1)-O(2) bond lengths are 1.96 Å. All Nb(1)-O(3) bond lengths are 2.10 Å. In the second Nb site, Nb(2) is bonded to three equivalent O(1) and three equivalent O(2) atoms to form distorted NbO6 octahedra that share corners with three equivalent Nb(1)O6 octahedra,  a faceface with one Ba(3)O12 cuboctahedra, and faces with three equivalent Ba(2)O12 cuboctahedra. The corner-sharing octahedral tilt angles are 9°. All Nb(2)-O(1) bond lengths are 1.87 Å. All Nb(2)-O(2) bond lengths are 2.27 Å. There are three inequivalent O sites. In the first O site, O(1) is bonded in a distorted single-bond geometry to one Ba(2), three equivalent Ba(1), and one Nb(2) atom. In the second O site, O(2) is bonded in a 2-coordinate geometry to one Ba(1), one Ba(3), two equivalent Ba(2), one Nb(1), and one Nb(2) atom. In the third O site, O(3) is bonded in a distorted linear geometry to two equivalent Ba(2), two equivalent Ba(3), and two equivalent Nb(1) atoms.