Cr8O21 crystallizes in the triclinic P-1 space group. There are four inequivalent Cr sites. In the first Cr site, Cr(1) is bonded to one O(10), one O(3), one O(5), and one O(8) atom to form corner-sharing CrO4 tetrahedra. In the second Cr site, Cr(2) is bonded to one O(11), one O(4), one O(6), one O(7), and two equivalent O(1) atoms to form CrO6 octahedra that share corners with two equivalent Cr(3)O4 tetrahedra, corners with four equivalent Cr(4)O4 tetrahedra, and  an edgeedge with one Cr(2)O6 octahedra. In the third Cr site, Cr(3) is bonded to one O(3), one O(6), one O(7), and one O(9) atom to form CrO4 tetrahedra that share corners with two equivalent Cr(2)O6 octahedra and  a cornercorner with one Cr(1)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 18-39°. In the fourth Cr site, Cr(4) is bonded to one O(1), one O(11), one O(2), and one O(4) atom to form corner-sharing CrO4 tetrahedra. The corner-sharing octahedral tilt angles range from 21-58°. There are eleven inequivalent O sites. In the first O site, O(1) is bonded in a trigonal planar geometry to one Cr(4) and two equivalent Cr(2) atoms. In the second O site, O(2) is bonded in a distorted water-like geometry to one Cr(4) and one O(9) atom. In the third O site, O(3) is bonded in a bent 150 degrees geometry to one Cr(1) and one Cr(3) atom. In the fourth O site, O(4) is bonded in a bent 150 degrees geometry to one Cr(2) and one Cr(4) atom. In the fifth O site, O(5) is bonded in a single-bond geometry to one Cr(1) atom. In the sixth O site, O(6) is bonded in a bent 150 degrees geometry to one Cr(2) and one Cr(3) atom. In the seventh O site, O(7) is bonded in a linear geometry to one Cr(2) and one Cr(3) atom. In the eighth O site, O(8) is bonded in a single-bond geometry to one Cr(1) atom. In the ninth O site, O(9) is bonded in a distorted single-bond geometry to one Cr(3) and one O(2) atom. In the tenth O site, O(10) is bonded in a linear geometry to two equivalent Cr(1) atoms. In the eleventh O site, O(11) is bonded in a bent 150 degrees geometry to one Cr(2) and one Cr(4) atom.
Cr8O21 crystallizes in the triclinic P-1 space group. There are four inequivalent Cr sites. In the first Cr site, Cr(1) is bonded to one O(10), one O(3), one O(5), and one O(8) atom to form corner-sharing CrO4 tetrahedra. The Cr(1)-O(10) bond length is 1.78 Å. The Cr(1)-O(3) bond length is 1.76 Å. The Cr(1)-O(5) bond length is 1.59 Å. The Cr(1)-O(8) bond length is 1.60 Å. In the second Cr site, Cr(2) is bonded to one O(11), one O(4), one O(6), one O(7), and two equivalent O(1) atoms to form CrO6 octahedra that share corners with two equivalent Cr(3)O4 tetrahedra, corners with four equivalent Cr(4)O4 tetrahedra, and  an edgeedge with one Cr(2)O6 octahedra. The Cr(2)-O(11) bond length is 1.96 Å. The Cr(2)-O(4) bond length is 1.96 Å. The Cr(2)-O(6) bond length is 1.96 Å. The Cr(2)-O(7) bond length is 2.03 Å. There is one shorter (2.01 Å) and one longer (2.08 Å) Cr(2)-O(1) bond length. In the third Cr site, Cr(3) is bonded to one O(3), one O(6), one O(7), and one O(9) atom to form CrO4 tetrahedra that share corners with two equivalent Cr(2)O6 octahedra and  a cornercorner with one Cr(1)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 18-39°. The Cr(3)-O(3) bond length is 1.79 Å. The Cr(3)-O(6) bond length is 1.70 Å. The Cr(3)-O(7) bond length is 1.67 Å. The Cr(3)-O(9) bond length is 1.81 Å. In the fourth Cr site, Cr(4) is bonded to one O(1), one O(11), one O(2), and one O(4) atom to form corner-sharing CrO4 tetrahedra. The corner-sharing octahedral tilt angles range from 21-58°. The Cr(4)-O(1) bond length is 1.80 Å. The Cr(4)-O(11) bond length is 1.67 Å. The Cr(4)-O(2) bond length is 1.85 Å. The Cr(4)-O(4) bond length is 1.67 Å. There are eleven inequivalent O sites. In the first O site, O(1) is bonded in a trigonal planar geometry to one Cr(4) and two equivalent Cr(2) atoms. In the second O site, O(2) is bonded in a distorted water-like geometry to one Cr(4) and one O(9) atom. The O(2)-O(9) bond length is 1.42 Å. In the third O site, O(3) is bonded in a bent 150 degrees geometry to one Cr(1) and one Cr(3) atom. In the fourth O site, O(4) is bonded in a bent 150 degrees geometry to one Cr(2) and one Cr(4) atom. In the fifth O site, O(5) is bonded in a single-bond geometry to one Cr(1) atom. In the sixth O site, O(6) is bonded in a bent 150 degrees geometry to one Cr(2) and one Cr(3) atom. In the seventh O site, O(7) is bonded in a linear geometry to one Cr(2) and one Cr(3) atom. In the eighth O site, O(8) is bonded in a single-bond geometry to one Cr(1) atom. In the ninth O site, O(9) is bonded in a distorted single-bond geometry to one Cr(3) and one O(2) atom. In the tenth O site, O(10) is bonded in a linear geometry to two equivalent Cr(1) atoms. In the eleventh O site, O(11) is bonded in a bent 150 degrees geometry to one Cr(2) and one Cr(4) atom.