CuNb3O8 crystallizes in the monoclinic P2_1/c space group. There are three inequivalent Nb sites. In the first Nb site, Nb(1) is bonded in a 6-coordinate geometry to one O(1), one O(3), one O(4), one O(6), and two equivalent O(7) atoms. In the second Nb site, Nb(2) is bonded to one O(1), one O(2), one O(3), one O(4), one O(7), and one O(8) atom to form distorted NbO6 octahedra that share corners with three equivalent Nb(3)O6 octahedra and corners with four equivalent Cu(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 44-59°. In the third Nb site, Nb(3) is bonded to one O(1), one O(2), one O(6), one O(8), and two equivalent O(5) atoms to form NbO6 octahedra that share  a cornercorner with one Cu(1)O6 octahedra, corners with two equivalent Nb(3)O6 octahedra, corners with three equivalent Nb(2)O6 octahedra, and edges with two equivalent Cu(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 44-51°. Cu(1) is bonded to one O(2), one O(3), one O(4), one O(5), one O(6), and one O(8) atom to form distorted CuO6 octahedra that share  a cornercorner with one Nb(3)O6 octahedra, corners with four equivalent Nb(2)O6 octahedra, and edges with two equivalent Nb(3)O6 octahedra. The corner-sharing octahedral tilt angles range from 51-59°. There are eight inequivalent O sites. In the first O site, O(5) is bonded in a 3-coordinate geometry to two equivalent Nb(3) and one Cu(1) atom. In the second O site, O(6) is bonded in a distorted trigonal planar geometry to one Nb(1), one Nb(3), and one Cu(1) atom. In the third O site, O(7) is bonded in a 3-coordinate geometry to one Nb(2) and two equivalent Nb(1) atoms. In the fourth O site, O(8) is bonded in a distorted trigonal planar geometry to one Nb(2), one Nb(3), and one Cu(1) atom. In the fifth O site, O(1) is bonded in a 3-coordinate geometry to one Nb(1), one Nb(2), and one Nb(3) atom. In the sixth O site, O(2) is bonded in a 3-coordinate geometry to one Nb(2), one Nb(3), and one Cu(1) atom. In the seventh O site, O(3) is bonded in a trigonal planar geometry to one Nb(1), one Nb(2), and one Cu(1) atom. In the eighth O site, O(4) is bonded in a trigonal planar geometry to one Nb(1), one Nb(2), and one Cu(1) atom.
CuNb3O8 crystallizes in the monoclinic P2_1/c space group. There are three inequivalent Nb sites. In the first Nb site, Nb(1) is bonded in a 6-coordinate geometry to one O(1), one O(3), one O(4), one O(6), and two equivalent O(7) atoms. The Nb(1)-O(1) bond length is 2.42 Å. The Nb(1)-O(3) bond length is 2.07 Å. The Nb(1)-O(4) bond length is 1.94 Å. The Nb(1)-O(6) bond length is 1.87 Å. There is one shorter (1.98 Å) and one longer (2.09 Å) Nb(1)-O(7) bond length. In the second Nb site, Nb(2) is bonded to one O(1), one O(2), one O(3), one O(4), one O(7), and one O(8) atom to form distorted NbO6 octahedra that share corners with three equivalent Nb(3)O6 octahedra and corners with four equivalent Cu(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 44-59°. The Nb(2)-O(1) bond length is 2.12 Å. The Nb(2)-O(2) bond length is 1.98 Å. The Nb(2)-O(3) bond length is 1.94 Å. The Nb(2)-O(4) bond length is 2.06 Å. The Nb(2)-O(7) bond length is 2.34 Å. The Nb(2)-O(8) bond length is 1.88 Å. In the third Nb site, Nb(3) is bonded to one O(1), one O(2), one O(6), one O(8), and two equivalent O(5) atoms to form NbO6 octahedra that share  a cornercorner with one Cu(1)O6 octahedra, corners with two equivalent Nb(3)O6 octahedra, corners with three equivalent Nb(2)O6 octahedra, and edges with two equivalent Cu(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 44-51°. The Nb(3)-O(1) bond length is 1.93 Å. The Nb(3)-O(2) bond length is 1.96 Å. The Nb(3)-O(6) bond length is 2.16 Å. The Nb(3)-O(8) bond length is 2.18 Å. There is one shorter (1.89 Å) and one longer (2.09 Å) Nb(3)-O(5) bond length. Cu(1) is bonded to one O(2), one O(3), one O(4), one O(5), one O(6), and one O(8) atom to form distorted CuO6 octahedra that share  a cornercorner with one Nb(3)O6 octahedra, corners with four equivalent Nb(2)O6 octahedra, and edges with two equivalent Nb(3)O6 octahedra. The corner-sharing octahedral tilt angles range from 51-59°. The Cu(1)-O(2) bond length is 2.47 Å. The Cu(1)-O(3) bond length is 2.10 Å. The Cu(1)-O(4) bond length is 2.10 Å. The Cu(1)-O(5) bond length is 2.40 Å. The Cu(1)-O(6) bond length is 2.16 Å. The Cu(1)-O(8) bond length is 2.12 Å. There are eight inequivalent O sites. In the first O site, O(5) is bonded in a 3-coordinate geometry to two equivalent Nb(3) and one Cu(1) atom. In the second O site, O(6) is bonded in a distorted trigonal planar geometry to one Nb(1), one Nb(3), and one Cu(1) atom. In the third O site, O(7) is bonded in a 3-coordinate geometry to one Nb(2) and two equivalent Nb(1) atoms. In the fourth O site, O(8) is bonded in a distorted trigonal planar geometry to one Nb(2), one Nb(3), and one Cu(1) atom. In the fifth O site, O(1) is bonded in a 3-coordinate geometry to one Nb(1), one Nb(2), and one Nb(3) atom. In the sixth O site, O(2) is bonded in a 3-coordinate geometry to one Nb(2), one Nb(3), and one Cu(1) atom. In the seventh O site, O(3) is bonded in a trigonal planar geometry to one Nb(1), one Nb(2), and one Cu(1) atom. In the eighth O site, O(4) is bonded in a trigonal planar geometry to one Nb(1), one Nb(2), and one Cu(1) atom.