Cu4As2O9 crystallizes in the triclinic P-1 space group. There are four inequivalent Cu sites. In the first Cu site, Cu(1) is bonded to one O(2), one O(4), one O(5), one O(7), and one O(9) atom to form distorted CuO5 trigonal bipyramids that share corners with two equivalent As(1)O4 tetrahedra, corners with two equivalent As(2)O4 tetrahedra,  a cornercorner with one Cu(4)O5 trigonal bipyramid,  an edgeedge with one Cu(2)O6 octahedra, and  an edgeedge with one Cu(4)O5 trigonal bipyramid. In the second Cu site, Cu(2) is bonded to one O(1), one O(3), one O(5), one O(7), and two equivalent O(6) atoms to form distorted CuO6 octahedra that share corners with two equivalent As(1)O4 tetrahedra, corners with three equivalent As(2)O4 tetrahedra,  an edgeedge with one Cu(2)O6 octahedra,  an edgeedge with one Cu(1)O5 trigonal bipyramid, and  an edgeedge with one Cu(4)O5 trigonal bipyramid. In the third Cu site, Cu(3) is bonded in a distorted rectangular see-saw-like geometry to one O(1), one O(7), one O(8), and two equivalent O(2) atoms. In the fourth Cu site, Cu(4) is bonded to one O(3), one O(4), one O(7), one O(8), and one O(9) atom to form distorted CuO5 trigonal bipyramids that share corners with two equivalent As(1)O4 tetrahedra, corners with two equivalent As(2)O4 tetrahedra,  a cornercorner with one Cu(1)O5 trigonal bipyramid,  an edgeedge with one Cu(2)O6 octahedra, and  an edgeedge with one Cu(1)O5 trigonal bipyramid. There are two inequivalent As sites. In the first As site, As(1) is bonded to one O(2), one O(6), one O(8), and one O(9) atom to form AsO4 tetrahedra that share corners with two equivalent Cu(2)O6 octahedra, corners with two equivalent Cu(1)O5 trigonal bipyramids, and corners with two equivalent Cu(4)O5 trigonal bipyramids. The corner-sharing octahedral tilt angles range from 56-58°. In the second As site, As(2) is bonded to one O(1), one O(3), one O(4), and one O(5) atom to form AsO4 tetrahedra that share corners with three equivalent Cu(2)O6 octahedra, corners with two equivalent Cu(1)O5 trigonal bipyramids, and corners with two equivalent Cu(4)O5 trigonal bipyramids. The corner-sharing octahedral tilt angles range from 44-60°. There are nine inequivalent O sites. In the first O site, O(1) is bonded in a distorted trigonal planar geometry to one Cu(2), one Cu(3), and one As(2) atom. In the second O site, O(2) is bonded in a 4-coordinate geometry to one Cu(1), two equivalent Cu(3), and one As(1) atom. In the third O site, O(3) is bonded in a 3-coordinate geometry to one Cu(2), one Cu(4), and one As(2) atom. In the fourth O site, O(4) is bonded in a trigonal non-coplanar geometry to one Cu(1), one Cu(4), and one As(2) atom. In the fifth O site, O(5) is bonded in a distorted trigonal planar geometry to one Cu(1), one Cu(2), and one As(2) atom. In the sixth O site, O(6) is bonded in a distorted trigonal planar geometry to two equivalent Cu(2) and one As(1) atom. In the seventh O site, O(7) is bonded in a tetrahedral geometry to one Cu(1), one Cu(2), one Cu(3), and one Cu(4) atom. In the eighth O site, O(8) is bonded in a distorted trigonal planar geometry to one Cu(3), one Cu(4), and one As(1) atom. In the ninth O site, O(9) is bonded in a 3-coordinate geometry to one Cu(1), one Cu(4), and one As(1) atom.
Cu4As2O9 crystallizes in the triclinic P-1 space group. There are four inequivalent Cu sites. In the first Cu site, Cu(1) is bonded to one O(2), one O(4), one O(5), one O(7), and one O(9) atom to form distorted CuO5 trigonal bipyramids that share corners with two equivalent As(1)O4 tetrahedra, corners with two equivalent As(2)O4 tetrahedra,  a cornercorner with one Cu(4)O5 trigonal bipyramid,  an edgeedge with one Cu(2)O6 octahedra, and  an edgeedge with one Cu(4)O5 trigonal bipyramid. The Cu(1)-O(2) bond length is 2.29 Å. The Cu(1)-O(4) bond length is 1.96 Å. The Cu(1)-O(5) bond length is 2.04 Å. The Cu(1)-O(7) bond length is 1.90 Å. The Cu(1)-O(9) bond length is 1.99 Å. In the second Cu site, Cu(2) is bonded to one O(1), one O(3), one O(5), one O(7), and two equivalent O(6) atoms to form distorted CuO6 octahedra that share corners with two equivalent As(1)O4 tetrahedra, corners with three equivalent As(2)O4 tetrahedra,  an edgeedge with one Cu(2)O6 octahedra,  an edgeedge with one Cu(1)O5 trigonal bipyramid, and  an edgeedge with one Cu(4)O5 trigonal bipyramid. The Cu(2)-O(1) bond length is 2.44 Å. The Cu(2)-O(3) bond length is 2.48 Å. The Cu(2)-O(5) bond length is 1.97 Å. The Cu(2)-O(7) bond length is 1.92 Å. There is one shorter (1.97 Å) and one longer (1.99 Å) Cu(2)-O(6) bond length. In the third Cu site, Cu(3) is bonded in a distorted rectangular see-saw-like geometry to one O(1), one O(7), one O(8), and two equivalent O(2) atoms. The Cu(3)-O(1) bond length is 1.93 Å. The Cu(3)-O(7) bond length is 1.93 Å. The Cu(3)-O(8) bond length is 1.94 Å. There is one shorter (1.95 Å) and one longer (2.65 Å) Cu(3)-O(2) bond length. In the fourth Cu site, Cu(4) is bonded to one O(3), one O(4), one O(7), one O(8), and one O(9) atom to form distorted CuO5 trigonal bipyramids that share corners with two equivalent As(1)O4 tetrahedra, corners with two equivalent As(2)O4 tetrahedra,  a cornercorner with one Cu(1)O5 trigonal bipyramid,  an edgeedge with one Cu(2)O6 octahedra, and  an edgeedge with one Cu(1)O5 trigonal bipyramid. The Cu(4)-O(3) bond length is 1.93 Å. The Cu(4)-O(4) bond length is 1.96 Å. The Cu(4)-O(7) bond length is 1.92 Å. The Cu(4)-O(8) bond length is 2.01 Å. The Cu(4)-O(9) bond length is 2.45 Å. There are two inequivalent As sites. In the first As site, As(1) is bonded to one O(2), one O(6), one O(8), and one O(9) atom to form AsO4 tetrahedra that share corners with two equivalent Cu(2)O6 octahedra, corners with two equivalent Cu(1)O5 trigonal bipyramids, and corners with two equivalent Cu(4)O5 trigonal bipyramids. The corner-sharing octahedral tilt angles range from 56-58°. The As(1)-O(2) bond length is 1.71 Å. The As(1)-O(6) bond length is 1.72 Å. The As(1)-O(8) bond length is 1.70 Å. The As(1)-O(9) bond length is 1.68 Å. In the second As site, As(2) is bonded to one O(1), one O(3), one O(4), and one O(5) atom to form AsO4 tetrahedra that share corners with three equivalent Cu(2)O6 octahedra, corners with two equivalent Cu(1)O5 trigonal bipyramids, and corners with two equivalent Cu(4)O5 trigonal bipyramids. The corner-sharing octahedral tilt angles range from 44-60°. The As(2)-O(1) bond length is 1.69 Å. The As(2)-O(3) bond length is 1.68 Å. The As(2)-O(4) bond length is 1.74 Å. The As(2)-O(5) bond length is 1.70 Å. There are nine inequivalent O sites. In the first O site, O(1) is bonded in a distorted trigonal planar geometry to one Cu(2), one Cu(3), and one As(2) atom. In the second O site, O(2) is bonded in a 4-coordinate geometry to one Cu(1), two equivalent Cu(3), and one As(1) atom. In the third O site, O(3) is bonded in a 3-coordinate geometry to one Cu(2), one Cu(4), and one As(2) atom. In the fourth O site, O(4) is bonded in a trigonal non-coplanar geometry to one Cu(1), one Cu(4), and one As(2) atom. In the fifth O site, O(5) is bonded in a distorted trigonal planar geometry to one Cu(1), one Cu(2), and one As(2) atom. In the sixth O site, O(6) is bonded in a distorted trigonal planar geometry to two equivalent Cu(2) and one As(1) atom. In the seventh O site, O(7) is bonded in a tetrahedral geometry to one Cu(1), one Cu(2), one Cu(3), and one Cu(4) atom. In the eighth O site, O(8) is bonded in a distorted trigonal planar geometry to one Cu(3), one Cu(4), and one As(1) atom. In the ninth O site, O(9) is bonded in a 3-coordinate geometry to one Cu(1), one Cu(4), and one As(1) atom.