CeNb3O9 crystallizes in the monoclinic C2/m space group. There are three inequivalent Ce sites. In the first Ce site, Ce(1) is bonded to two equivalent O(10), two equivalent O(7), four equivalent O(11), and four equivalent O(3) atoms to form CeO12 cuboctahedra that share faces with two equivalent Ce(2)O12 cuboctahedra, faces with two equivalent Ce(3)O12 cuboctahedra, faces with four equivalent Nb(2)O6 octahedra, and faces with four equivalent Nb(3)O6 octahedra. In the second Ce site, Ce(2) is bonded to two equivalent O(6), two equivalent O(8), four equivalent O(11), and four equivalent O(3) atoms to form CeO12 cuboctahedra that share corners with four equivalent Ce(3)O12 cuboctahedra, faces with two equivalent Ce(1)O12 cuboctahedra, faces with four equivalent Nb(2)O6 octahedra, and faces with four equivalent Nb(3)O6 octahedra. In the third Ce site, Ce(3) is bonded to one O(10), one O(5), one O(7), one O(9), two equivalent O(12), two equivalent O(13), two equivalent O(3), and two equivalent O(4) atoms to form CeO12 cuboctahedra that share corners with two equivalent Ce(2)O12 cuboctahedra, corners with two equivalent Ce(3)O12 cuboctahedra,  a faceface with one Ce(1)O12 cuboctahedra, faces with two equivalent Nb(2)O6 octahedra, faces with two equivalent Nb(3)O6 octahedra, and faces with four equivalent Nb(1)O6 octahedra. There are three inequivalent Nb sites. In the first Nb site, Nb(1) is bonded to one O(1), one O(12), one O(13), one O(4), one O(5), and one O(9) atom to form NbO6 octahedra that share  a cornercorner with one Nb(2)O6 octahedra, corners with two equivalent Nb(3)O6 octahedra, corners with three equivalent Nb(1)O6 octahedra, and faces with two equivalent Ce(3)O12 cuboctahedra. The corner-sharing octahedral tilt angles range from 0-14°. In the second Nb site, Nb(2) is bonded to one O(10), one O(11), one O(12), one O(2), one O(3), and one O(6) atom to form NbO6 octahedra that share  a cornercorner with one Nb(1)O6 octahedra, corners with two equivalent Nb(3)O6 octahedra, corners with three equivalent Nb(2)O6 octahedra,  a faceface with one Ce(1)O12 cuboctahedra,  a faceface with one Ce(2)O12 cuboctahedra, and  a faceface with one Ce(3)O12 cuboctahedra. The corner-sharing octahedral tilt angles range from 0-20°. In the third Nb site, Nb(3) is bonded to one O(1), one O(11), one O(13), one O(3), one O(7), and one O(8) atom to form NbO6 octahedra that share corners with two equivalent Nb(1)O6 octahedra, corners with two equivalent Nb(2)O6 octahedra, corners with two equivalent Nb(3)O6 octahedra,  a faceface with one Ce(1)O12 cuboctahedra,  a faceface with one Ce(2)O12 cuboctahedra, and  a faceface with one Ce(3)O12 cuboctahedra. The corner-sharing octahedral tilt angles range from 2-22°. There are thirteen inequivalent O sites. In the first O site, O(1) is bonded in a linear geometry to one Nb(1) and one Nb(3) atom. In the second O site, O(2) is bonded in a linear geometry to two equivalent Nb(2) atoms. In the third O site, O(3) is bonded in a 5-coordinate geometry to one Ce(1), one Ce(2), one Ce(3), one Nb(2), and one Nb(3) atom. In the fourth O site, O(4) is bonded in a distorted linear geometry to two equivalent Ce(3) and two equivalent Nb(1) atoms. In the fifth O site, O(5) is bonded in a distorted T-shaped geometry to one Ce(3) and two equivalent Nb(1) atoms. In the sixth O site, O(6) is bonded in a distorted T-shaped geometry to one Ce(2) and two equivalent Nb(2) atoms. In the seventh O site, O(7) is bonded in a 4-coordinate geometry to one Ce(1), one Ce(3), and two equivalent Nb(3) atoms. In the eighth O site, O(8) is bonded in a distorted T-shaped geometry to one Ce(2) and two equivalent Nb(3) atoms. In the ninth O site, O(9) is bonded in a distorted T-shaped geometry to one Ce(3) and two equivalent Nb(1) atoms. In the tenth O site, O(10) is bonded in a 4-coordinate geometry to one Ce(1), one Ce(3), and two equivalent Nb(2) atoms. In the eleventh O site, O(11) is bonded in a distorted rectangular see-saw-like geometry to one Ce(1), one Ce(2), one Nb(2), and one Nb(3) atom. In the twelfth O site, O(12) is bonded in a distorted T-shaped geometry to one Ce(3), one Nb(1), and one Nb(2) atom. In the thirteenth O site, O(13) is bonded in a distorted T-shaped geometry to one Ce(3), one Nb(1), and one Nb(3) atom.
CeNb3O9 crystallizes in the monoclinic C2/m space group. There are three inequivalent Ce sites. In the first Ce site, Ce(1) is bonded to two equivalent O(10), two equivalent O(7), four equivalent O(11), and four equivalent O(3) atoms to form CeO12 cuboctahedra that share faces with two equivalent Ce(2)O12 cuboctahedra, faces with two equivalent Ce(3)O12 cuboctahedra, faces with four equivalent Nb(2)O6 octahedra, and faces with four equivalent Nb(3)O6 octahedra. Both Ce(1)-O(10) bond lengths are 2.61 Å. Both Ce(1)-O(7) bond lengths are 2.60 Å. All Ce(1)-O(11) bond lengths are 2.72 Å. All Ce(1)-O(3) bond lengths are 2.68 Å. In the second Ce site, Ce(2) is bonded to two equivalent O(6), two equivalent O(8), four equivalent O(11), and four equivalent O(3) atoms to form CeO12 cuboctahedra that share corners with four equivalent Ce(3)O12 cuboctahedra, faces with two equivalent Ce(1)O12 cuboctahedra, faces with four equivalent Nb(2)O6 octahedra, and faces with four equivalent Nb(3)O6 octahedra. Both Ce(2)-O(6) bond lengths are 2.67 Å. Both Ce(2)-O(8) bond lengths are 2.66 Å. All Ce(2)-O(11) bond lengths are 2.68 Å. All Ce(2)-O(3) bond lengths are 2.77 Å. In the third Ce site, Ce(3) is bonded to one O(10), one O(5), one O(7), one O(9), two equivalent O(12), two equivalent O(13), two equivalent O(3), and two equivalent O(4) atoms to form CeO12 cuboctahedra that share corners with two equivalent Ce(2)O12 cuboctahedra, corners with two equivalent Ce(3)O12 cuboctahedra,  a faceface with one Ce(1)O12 cuboctahedra, faces with two equivalent Nb(2)O6 octahedra, faces with two equivalent Nb(3)O6 octahedra, and faces with four equivalent Nb(1)O6 octahedra. The Ce(3)-O(10) bond length is 2.74 Å. The Ce(3)-O(5) bond length is 2.64 Å. The Ce(3)-O(7) bond length is 2.74 Å. The Ce(3)-O(9) bond length is 2.62 Å. Both Ce(3)-O(12) bond lengths are 2.64 Å. Both Ce(3)-O(13) bond lengths are 2.63 Å. Both Ce(3)-O(3) bond lengths are 2.90 Å. Both Ce(3)-O(4) bond lengths are 2.77 Å. There are three inequivalent Nb sites. In the first Nb site, Nb(1) is bonded to one O(1), one O(12), one O(13), one O(4), one O(5), and one O(9) atom to form NbO6 octahedra that share  a cornercorner with one Nb(2)O6 octahedra, corners with two equivalent Nb(3)O6 octahedra, corners with three equivalent Nb(1)O6 octahedra, and faces with two equivalent Ce(3)O12 cuboctahedra. The corner-sharing octahedral tilt angles range from 0-14°. The Nb(1)-O(1) bond length is 1.97 Å. The Nb(1)-O(12) bond length is 2.03 Å. The Nb(1)-O(13) bond length is 2.03 Å. The Nb(1)-O(4) bond length is 2.03 Å. The Nb(1)-O(5) bond length is 1.99 Å. The Nb(1)-O(9) bond length is 1.99 Å. In the second Nb site, Nb(2) is bonded to one O(10), one O(11), one O(12), one O(2), one O(3), and one O(6) atom to form NbO6 octahedra that share  a cornercorner with one Nb(1)O6 octahedra, corners with two equivalent Nb(3)O6 octahedra, corners with three equivalent Nb(2)O6 octahedra,  a faceface with one Ce(1)O12 cuboctahedra,  a faceface with one Ce(2)O12 cuboctahedra, and  a faceface with one Ce(3)O12 cuboctahedra. The corner-sharing octahedral tilt angles range from 0-20°. The Nb(2)-O(10) bond length is 2.03 Å. The Nb(2)-O(11) bond length is 2.06 Å. The Nb(2)-O(12) bond length is 1.96 Å. The Nb(2)-O(2) bond length is 1.94 Å. The Nb(2)-O(3) bond length is 2.09 Å. The Nb(2)-O(6) bond length is 1.97 Å. In the third Nb site, Nb(3) is bonded to one O(1), one O(11), one O(13), one O(3), one O(7), and one O(8) atom to form NbO6 octahedra that share corners with two equivalent Nb(1)O6 octahedra, corners with two equivalent Nb(2)O6 octahedra, corners with two equivalent Nb(3)O6 octahedra,  a faceface with one Ce(1)O12 cuboctahedra,  a faceface with one Ce(2)O12 cuboctahedra, and  a faceface with one Ce(3)O12 cuboctahedra. The corner-sharing octahedral tilt angles range from 2-22°. The Nb(3)-O(1) bond length is 1.92 Å. The Nb(3)-O(11) bond length is 2.05 Å. The Nb(3)-O(13) bond length is 1.96 Å. The Nb(3)-O(3) bond length is 2.13 Å. The Nb(3)-O(7) bond length is 2.04 Å. The Nb(3)-O(8) bond length is 1.97 Å. There are thirteen inequivalent O sites. In the first O site, O(1) is bonded in a linear geometry to one Nb(1) and one Nb(3) atom. In the second O site, O(2) is bonded in a linear geometry to two equivalent Nb(2) atoms. In the third O site, O(3) is bonded in a 5-coordinate geometry to one Ce(1), one Ce(2), one Ce(3), one Nb(2), and one Nb(3) atom. In the fourth O site, O(4) is bonded in a distorted linear geometry to two equivalent Ce(3) and two equivalent Nb(1) atoms. In the fifth O site, O(5) is bonded in a distorted T-shaped geometry to one Ce(3) and two equivalent Nb(1) atoms. In the sixth O site, O(6) is bonded in a distorted T-shaped geometry to one Ce(2) and two equivalent Nb(2) atoms. In the seventh O site, O(7) is bonded in a 4-coordinate geometry to one Ce(1), one Ce(3), and two equivalent Nb(3) atoms. In the eighth O site, O(8) is bonded in a distorted T-shaped geometry to one Ce(2) and two equivalent Nb(3) atoms. In the ninth O site, O(9) is bonded in a distorted T-shaped geometry to one Ce(3) and two equivalent Nb(1) atoms. In the tenth O site, O(10) is bonded in a 4-coordinate geometry to one Ce(1), one Ce(3), and two equivalent Nb(2) atoms. In the eleventh O site, O(11) is bonded in a distorted rectangular see-saw-like geometry to one Ce(1), one Ce(2), one Nb(2), and one Nb(3) atom. In the twelfth O site, O(12) is bonded in a distorted T-shaped geometry to one Ce(3), one Nb(1), and one Nb(2) atom. In the thirteenth O site, O(13) is bonded in a distorted T-shaped geometry to one Ce(3), one Nb(1), and one Nb(3) atom.