BaNb5O8 crystallizes in the tetragonal P4/m space group. Ba(1) is bonded to four equivalent O(2) and eight equivalent O(1) atoms to form BaO12 cuboctahedra that share corners with eight equivalent Nb(1)O5 square pyramids, faces with two equivalent Ba(1)O12 cuboctahedra, and faces with eight equivalent Nb(1)O5 square pyramids. There are two inequivalent Nb sites. In the first Nb site, Nb(1) is bonded to two equivalent O(2) and three equivalent O(1) atoms to form NbO5 square pyramids that share corners with two equivalent Ba(1)O12 cuboctahedra, corners with six equivalent Nb(1)O5 square pyramids,  an edgeedge with one Nb(1)O5 square pyramid, and faces with two equivalent Ba(1)O12 cuboctahedra. In the second Nb site, Nb(2) is bonded in a square co-planar geometry to four equivalent O(2) atoms. There are two inequivalent O sites. In the first O site, O(1) is bonded in a distorted T-shaped geometry to two equivalent Ba(1) and three equivalent Nb(1) atoms. In the second O site, O(2) is bonded in a distorted rectangular see-saw-like geometry to one Ba(1), one Nb(2), and two equivalent Nb(1) atoms.
BaNb5O8 crystallizes in the tetragonal P4/m space group. Ba(1) is bonded to four equivalent O(2) and eight equivalent O(1) atoms to form BaO12 cuboctahedra that share corners with eight equivalent Nb(1)O5 square pyramids, faces with two equivalent Ba(1)O12 cuboctahedra, and faces with eight equivalent Nb(1)O5 square pyramids. All Ba(1)-O(2) bond lengths are 2.92 Å. All Ba(1)-O(1) bond lengths are 2.96 Å. There are two inequivalent Nb sites. In the first Nb site, Nb(1) is bonded to two equivalent O(2) and three equivalent O(1) atoms to form NbO5 square pyramids that share corners with two equivalent Ba(1)O12 cuboctahedra, corners with six equivalent Nb(1)O5 square pyramids,  an edgeedge with one Nb(1)O5 square pyramid, and faces with two equivalent Ba(1)O12 cuboctahedra. Both Nb(1)-O(2) bond lengths are 2.08 Å. There are a spread of Nb(1)-O(1) bond distances ranging from 2.10-2.18 Å. In the second Nb site, Nb(2) is bonded in a square co-planar geometry to four equivalent O(2) atoms. All Nb(2)-O(2) bond lengths are 2.12 Å. There are two inequivalent O sites. In the first O site, O(1) is bonded in a distorted T-shaped geometry to two equivalent Ba(1) and three equivalent Nb(1) atoms. In the second O site, O(2) is bonded in a distorted rectangular see-saw-like geometry to one Ba(1), one Nb(2), and two equivalent Nb(1) atoms.