TaNbCu3O8 crystallizes in the triclinic P1 space group. Ta(1) is bonded to one O(1), one O(3), one O(5), one O(6), one O(7), and one O(8) atom to form distorted TaO6 octahedra that share corners with two equivalent Cu(3)O5 square pyramids, corners with three equivalent Cu(2)O5 square pyramids, edges with two equivalent Nb(1)O6 octahedra, and  an edgeedge with one Cu(3)O5 square pyramid. Nb(1) is bonded to one O(2), one O(4), one O(5), one O(6), one O(7), and one O(8) atom to form distorted NbO6 octahedra that share corners with two equivalent Cu(2)O5 square pyramids, corners with three equivalent Cu(3)O5 square pyramids, edges with two equivalent Ta(1)O6 octahedra, and  an edgeedge with one Cu(2)O5 square pyramid. There are three inequivalent Cu sites. In the first Cu site, Cu(1) is bonded in a square co-planar geometry to one O(1), one O(2), one O(3), and one O(4) atom. In the second Cu site, Cu(2) is bonded to one O(1), one O(2), one O(4), one O(5), and one O(7) atom to form distorted CuO5 square pyramids that share corners with two equivalent Nb(1)O6 octahedra, corners with three equivalent Ta(1)O6 octahedra,  an edgeedge with one Nb(1)O6 octahedra, and  an edgeedge with one Cu(3)O5 square pyramid. The corner-sharing octahedral tilt angles range from 37-56°. In the third Cu site, Cu(3) is bonded to one O(1), one O(2), one O(3), one O(6), and one O(8) atom to form distorted CuO5 square pyramids that share corners with two equivalent Ta(1)O6 octahedra, corners with three equivalent Nb(1)O6 octahedra,  an edgeedge with one Ta(1)O6 octahedra, and  an edgeedge with one Cu(2)O5 square pyramid. The corner-sharing octahedral tilt angles range from 39-57°. There are eight inequivalent O sites. In the first O site, O(1) is bonded to one Ta(1), one Cu(1), one Cu(2), and one Cu(3) atom to form a mixture of corner and edge-sharing OTaCu3 trigonal pyramids. In the second O site, O(2) is bonded to one Nb(1), one Cu(1), one Cu(2), and one Cu(3) atom to form a mixture of corner and edge-sharing ONbCu3 trigonal pyramids. In the third O site, O(3) is bonded in a distorted trigonal planar geometry to one Ta(1), one Cu(1), and one Cu(3) atom. In the fourth O site, O(4) is bonded in a distorted trigonal planar geometry to one Nb(1), one Cu(1), and one Cu(2) atom. In the fifth O site, O(5) is bonded in a distorted trigonal planar geometry to one Ta(1), one Nb(1), and one Cu(2) atom. In the sixth O site, O(6) is bonded in a distorted trigonal planar geometry to one Ta(1), one Nb(1), and one Cu(3) atom. In the seventh O site, O(7) is bonded in a distorted T-shaped geometry to one Ta(1), one Nb(1), and one Cu(2) atom. In the eighth O site, O(8) is bonded in a distorted T-shaped geometry to one Ta(1), one Nb(1), and one Cu(3) atom.
TaNbCu3O8 crystallizes in the triclinic P1 space group. Ta(1) is bonded to one O(1), one O(3), one O(5), one O(6), one O(7), and one O(8) atom to form distorted TaO6 octahedra that share corners with two equivalent Cu(3)O5 square pyramids, corners with three equivalent Cu(2)O5 square pyramids, edges with two equivalent Nb(1)O6 octahedra, and  an edgeedge with one Cu(3)O5 square pyramid. The Ta(1)-O(1) bond length is 2.01 Å. The Ta(1)-O(3) bond length is 1.88 Å. The Ta(1)-O(5) bond length is 2.28 Å. The Ta(1)-O(6) bond length is 1.91 Å. The Ta(1)-O(7) bond length is 1.99 Å. The Ta(1)-O(8) bond length is 2.09 Å. Nb(1) is bonded to one O(2), one O(4), one O(5), one O(6), one O(7), and one O(8) atom to form distorted NbO6 octahedra that share corners with two equivalent Cu(2)O5 square pyramids, corners with three equivalent Cu(3)O5 square pyramids, edges with two equivalent Ta(1)O6 octahedra, and  an edgeedge with one Cu(2)O5 square pyramid. The Nb(1)-O(2) bond length is 2.02 Å. The Nb(1)-O(4) bond length is 1.88 Å. The Nb(1)-O(5) bond length is 1.93 Å. The Nb(1)-O(6) bond length is 2.34 Å. The Nb(1)-O(7) bond length is 2.12 Å. The Nb(1)-O(8) bond length is 2.00 Å. There are three inequivalent Cu sites. In the first Cu site, Cu(1) is bonded in a square co-planar geometry to one O(1), one O(2), one O(3), and one O(4) atom. The Cu(1)-O(1) bond length is 1.93 Å. The Cu(1)-O(2) bond length is 1.93 Å. The Cu(1)-O(3) bond length is 1.96 Å. The Cu(1)-O(4) bond length is 1.96 Å. In the second Cu site, Cu(2) is bonded to one O(1), one O(2), one O(4), one O(5), and one O(7) atom to form distorted CuO5 square pyramids that share corners with two equivalent Nb(1)O6 octahedra, corners with three equivalent Ta(1)O6 octahedra,  an edgeedge with one Nb(1)O6 octahedra, and  an edgeedge with one Cu(3)O5 square pyramid. The corner-sharing octahedral tilt angles range from 37-56°. The Cu(2)-O(1) bond length is 2.32 Å. The Cu(2)-O(2) bond length is 1.98 Å. The Cu(2)-O(4) bond length is 2.02 Å. The Cu(2)-O(5) bond length is 1.96 Å. The Cu(2)-O(7) bond length is 2.02 Å. In the third Cu site, Cu(3) is bonded to one O(1), one O(2), one O(3), one O(6), and one O(8) atom to form distorted CuO5 square pyramids that share corners with two equivalent Ta(1)O6 octahedra, corners with three equivalent Nb(1)O6 octahedra,  an edgeedge with one Ta(1)O6 octahedra, and  an edgeedge with one Cu(2)O5 square pyramid. The corner-sharing octahedral tilt angles range from 39-57°. The Cu(3)-O(1) bond length is 1.99 Å. The Cu(3)-O(2) bond length is 2.30 Å. The Cu(3)-O(3) bond length is 2.02 Å. The Cu(3)-O(6) bond length is 1.95 Å. The Cu(3)-O(8) bond length is 2.03 Å. There are eight inequivalent O sites. In the first O site, O(1) is bonded to one Ta(1), one Cu(1), one Cu(2), and one Cu(3) atom to form a mixture of corner and edge-sharing OTaCu3 trigonal pyramids. In the second O site, O(2) is bonded to one Nb(1), one Cu(1), one Cu(2), and one Cu(3) atom to form a mixture of corner and edge-sharing ONbCu3 trigonal pyramids. In the third O site, O(3) is bonded in a distorted trigonal planar geometry to one Ta(1), one Cu(1), and one Cu(3) atom. In the fourth O site, O(4) is bonded in a distorted trigonal planar geometry to one Nb(1), one Cu(1), and one Cu(2) atom. In the fifth O site, O(5) is bonded in a distorted trigonal planar geometry to one Ta(1), one Nb(1), and one Cu(2) atom. In the sixth O site, O(6) is bonded in a distorted trigonal planar geometry to one Ta(1), one Nb(1), and one Cu(3) atom. In the seventh O site, O(7) is bonded in a distorted T-shaped geometry to one Ta(1), one Nb(1), and one Cu(2) atom. In the eighth O site, O(8) is bonded in a distorted T-shaped geometry to one Ta(1), one Nb(1), and one Cu(3) atom.