Eu2Nb5O9 crystallizes in the tetragonal P4/mmm space group. Eu(1) is bonded to four equivalent O(1), four equivalent O(2), and four equivalent O(3) atoms to form EuO12 cuboctahedra that share corners with eight equivalent Eu(1)O12 cuboctahedra, faces with five equivalent Eu(1)O12 cuboctahedra, faces with four equivalent Nb(1)O6 octahedra, and faces with four equivalent Nb(2)O5 square pyramids. There are three inequivalent Nb sites. In the first Nb site, Nb(1) is bonded to two equivalent O(2) and four equivalent O(3) atoms to form NbO6 octahedra that share corners with four equivalent Nb(1)O6 octahedra, corners with two equivalent Nb(2)O5 square pyramids, and faces with eight equivalent Eu(1)O12 cuboctahedra. The corner-sharing octahedra are not tilted. In the second Nb site, Nb(2) is bonded to one O(2) and four equivalent O(1) atoms to form NbO5 square pyramids that share  a cornercorner with one Nb(1)O6 octahedra, corners with four equivalent Nb(2)O5 square pyramids, and faces with four equivalent Eu(1)O12 cuboctahedra. The corner-sharing octahedra are not tilted. In the third Nb site, Nb(3) is bonded in a square co-planar geometry to two equivalent O(1) and two equivalent O(4) atoms. There are four inequivalent O sites. In the first O site, O(1) is bonded to two equivalent Eu(1), one Nb(3), and two equivalent Nb(2) atoms to form a mixture of distorted edge and corner-sharing OEu2Nb3 trigonal bipyramids. In the second O site, O(2) is bonded in a distorted linear geometry to four equivalent Eu(1), one Nb(1), and one Nb(2) atom. In the third O site, O(3) is bonded in a distorted linear geometry to four equivalent Eu(1) and two equivalent Nb(1) atoms. In the fourth O site, O(4) is bonded in a square co-planar geometry to four equivalent Nb(3) atoms.
Eu2Nb5O9 crystallizes in the tetragonal P4/mmm space group. Eu(1) is bonded to four equivalent O(1), four equivalent O(2), and four equivalent O(3) atoms to form EuO12 cuboctahedra that share corners with eight equivalent Eu(1)O12 cuboctahedra, faces with five equivalent Eu(1)O12 cuboctahedra, faces with four equivalent Nb(1)O6 octahedra, and faces with four equivalent Nb(2)O5 square pyramids. All Eu(1)-O(1) bond lengths are 2.76 Å. All Eu(1)-O(2) bond lengths are 2.96 Å. All Eu(1)-O(3) bond lengths are 2.94 Å. There are three inequivalent Nb sites. In the first Nb site, Nb(1) is bonded to two equivalent O(2) and four equivalent O(3) atoms to form NbO6 octahedra that share corners with four equivalent Nb(1)O6 octahedra, corners with two equivalent Nb(2)O5 square pyramids, and faces with eight equivalent Eu(1)O12 cuboctahedra. The corner-sharing octahedra are not tilted. Both Nb(1)-O(2) bond lengths are 1.96 Å. All Nb(1)-O(3) bond lengths are 2.09 Å. In the second Nb site, Nb(2) is bonded to one O(2) and four equivalent O(1) atoms to form NbO5 square pyramids that share  a cornercorner with one Nb(1)O6 octahedra, corners with four equivalent Nb(2)O5 square pyramids, and faces with four equivalent Eu(1)O12 cuboctahedra. The corner-sharing octahedra are not tilted. The Nb(2)-O(2) bond length is 2.10 Å. All Nb(2)-O(1) bond lengths are 2.10 Å. In the third Nb site, Nb(3) is bonded in a square co-planar geometry to two equivalent O(1) and two equivalent O(4) atoms. Both Nb(3)-O(1) bond lengths are 2.17 Å. Both Nb(3)-O(4) bond lengths are 2.09 Å. There are four inequivalent O sites. In the first O site, O(1) is bonded to two equivalent Eu(1), one Nb(3), and two equivalent Nb(2) atoms to form a mixture of distorted edge and corner-sharing OEu2Nb3 trigonal bipyramids. In the second O site, O(2) is bonded in a distorted linear geometry to four equivalent Eu(1), one Nb(1), and one Nb(2) atom. In the third O site, O(3) is bonded in a distorted linear geometry to four equivalent Eu(1) and two equivalent Nb(1) atoms. In the fourth O site, O(4) is bonded in a square co-planar geometry to four equivalent Nb(3) atoms.