K6Cr2O9 crystallizes in the monoclinic Cm space group. There are four inequivalent K sites. In the first K site, K(1) is bonded in a 5-coordinate geometry to one O(1), one O(5), one O(6), and two equivalent O(2) atoms. In the second K site, K(2) is bonded in a 5-coordinate geometry to one O(1), one O(6), one O(7), and two equivalent O(4) atoms. In the third K site, K(3) is bonded in a 6-coordinate geometry to one O(1), one O(3), one O(5), one O(6), and two equivalent O(2) atoms. In the fourth K site, K(4) is bonded to one O(3), one O(7), two equivalent O(2), and two equivalent O(4) atoms to form KO6 pentagonal pyramids that share  a cornercorner with one Cr(2)O4 tetrahedra, corners with three equivalent Cr(1)O4 tetrahedra, and  an edgeedge with one Cr(2)O4 tetrahedra. There are two inequivalent Cr sites. In the first Cr site, Cr(1) is bonded to one O(3), one O(5), and two equivalent O(4) atoms to form CrO4 tetrahedra that share corners with three equivalent K(4)O6 pentagonal pyramids. In the second Cr site, Cr(2) is bonded to one O(1), one O(7), and two equivalent O(2) atoms to form CrO4 tetrahedra that share  a cornercorner with one K(4)O6 pentagonal pyramid and  an edgeedge with one K(4)O6 pentagonal pyramid. There are seven inequivalent O sites. In the first O site, O(1) is bonded in a 1-coordinate geometry to one K(1), two equivalent K(2), two equivalent K(3), and one Cr(2) atom. In the second O site, O(2) is bonded in a 5-coordinate geometry to one K(1), one K(4), two equivalent K(3), and one Cr(2) atom. In the third O site, O(3) is bonded in a 1-coordinate geometry to one K(4), two equivalent K(3), and one Cr(1) atom. In the fourth O site, O(4) is bonded in a 4-coordinate geometry to one K(4), two equivalent K(2), and one Cr(1) atom. In the fifth O site, O(5) is bonded in a 4-coordinate geometry to one K(1), two equivalent K(3), and one Cr(1) atom. In the sixth O site, O(6) is bonded in a 6-coordinate geometry to one K(1), two equivalent K(2), two equivalent K(3), and one O(7) atom. In the seventh O site, O(7) is bonded in a distorted single-bond geometry to one K(4), two equivalent K(2), one Cr(2), and one O(6) atom.
K6Cr2O9 crystallizes in the monoclinic Cm space group. There are four inequivalent K sites. In the first K site, K(1) is bonded in a 5-coordinate geometry to one O(1), one O(5), one O(6), and two equivalent O(2) atoms. The K(1)-O(1) bond length is 2.91 Å. The K(1)-O(5) bond length is 2.90 Å. The K(1)-O(6) bond length is 2.65 Å. Both K(1)-O(2) bond lengths are 2.97 Å. In the second K site, K(2) is bonded in a 5-coordinate geometry to one O(1), one O(6), one O(7), and two equivalent O(4) atoms. The K(2)-O(1) bond length is 2.70 Å. The K(2)-O(6) bond length is 2.65 Å. The K(2)-O(7) bond length is 2.79 Å. There is one shorter (2.66 Å) and one longer (2.68 Å) K(2)-O(4) bond length. In the third K site, K(3) is bonded in a 6-coordinate geometry to one O(1), one O(3), one O(5), one O(6), and two equivalent O(2) atoms. The K(3)-O(1) bond length is 3.14 Å. The K(3)-O(3) bond length is 2.69 Å. The K(3)-O(5) bond length is 2.59 Å. The K(3)-O(6) bond length is 3.13 Å. There is one shorter (2.59 Å) and one longer (2.60 Å) K(3)-O(2) bond length. In the fourth K site, K(4) is bonded to one O(3), one O(7), two equivalent O(2), and two equivalent O(4) atoms to form KO6 pentagonal pyramids that share  a cornercorner with one Cr(2)O4 tetrahedra, corners with three equivalent Cr(1)O4 tetrahedra, and  an edgeedge with one Cr(2)O4 tetrahedra. The K(4)-O(3) bond length is 3.09 Å. The K(4)-O(7) bond length is 3.08 Å. Both K(4)-O(2) bond lengths are 3.02 Å. Both K(4)-O(4) bond lengths are 3.03 Å. There are two inequivalent Cr sites. In the first Cr site, Cr(1) is bonded to one O(3), one O(5), and two equivalent O(4) atoms to form CrO4 tetrahedra that share corners with three equivalent K(4)O6 pentagonal pyramids. The Cr(1)-O(3) bond length is 1.75 Å. The Cr(1)-O(5) bond length is 1.74 Å. Both Cr(1)-O(4) bond lengths are 1.74 Å. In the second Cr site, Cr(2) is bonded to one O(1), one O(7), and two equivalent O(2) atoms to form CrO4 tetrahedra that share  a cornercorner with one K(4)O6 pentagonal pyramid and  an edgeedge with one K(4)O6 pentagonal pyramid. The Cr(2)-O(1) bond length is 1.74 Å. The Cr(2)-O(7) bond length is 1.81 Å. Both Cr(2)-O(2) bond lengths are 1.71 Å. There are seven inequivalent O sites. In the first O site, O(1) is bonded in a 1-coordinate geometry to one K(1), two equivalent K(2), two equivalent K(3), and one Cr(2) atom. In the second O site, O(2) is bonded in a 5-coordinate geometry to one K(1), one K(4), two equivalent K(3), and one Cr(2) atom. In the third O site, O(3) is bonded in a 1-coordinate geometry to one K(4), two equivalent K(3), and one Cr(1) atom. In the fourth O site, O(4) is bonded in a 4-coordinate geometry to one K(4), two equivalent K(2), and one Cr(1) atom. In the fifth O site, O(5) is bonded in a 4-coordinate geometry to one K(1), two equivalent K(3), and one Cr(1) atom. In the sixth O site, O(6) is bonded in a 6-coordinate geometry to one K(1), two equivalent K(2), two equivalent K(3), and one O(7) atom. The O(6)-O(7) bond length is 1.48 Å. In the seventh O site, O(7) is bonded in a distorted single-bond geometry to one K(4), two equivalent K(2), one Cr(2), and one O(6) atom.