Cs3Bi2I9 crystallizes in the monoclinic C2/c space group. There are two inequivalent Cs sites. In the first Cs site, Cs(1) is bonded in a distorted q6 geometry to two equivalent I(3), two equivalent I(4), two equivalent I(5), and four equivalent I(2) atoms. In the second Cs site, Cs(2) is bonded in a 11-coordinate geometry to one I(2), one I(5), three equivalent I(1), three equivalent I(3), and three equivalent I(4) atoms. Bi(1) is bonded to one I(1), one I(3), one I(4), one I(5), and two equivalent I(2) atoms to form face-sharing BiI6 octahedra. There are five inequivalent I sites. In the first I site, I(5) is bonded in a 6-coordinate geometry to two equivalent Cs(1), two equivalent Cs(2), and two equivalent Bi(1) atoms. In the second I site, I(1) is bonded in a distorted rectangular see-saw-like geometry to three equivalent Cs(2) and one Bi(1) atom. In the third I site, I(2) is bonded to one Cs(2), two equivalent Cs(1), and two equivalent Bi(1) atoms to form ICs3Bi2 square pyramids that share corners with three equivalent I(4)Cs4Bi square pyramids, corners with four equivalent I(3)Cs4Bi square pyramids, corners with five equivalent I(2)Cs3Bi2 square pyramids,  an edgeedge with one I(3)Cs4Bi square pyramid, edges with three equivalent I(4)Cs4Bi square pyramids,  a faceface with one I(2)Cs3Bi2 square pyramid, and  a faceface with one I(3)Cs4Bi square pyramid. In the fourth I site, I(3) is bonded to one Cs(1), three equivalent Cs(2), and one Bi(1) atom to form distorted ICs4Bi square pyramids that share corners with three equivalent I(3)Cs4Bi square pyramids, corners with four equivalent I(2)Cs3Bi2 square pyramids, corners with five equivalent I(4)Cs4Bi square pyramids,  an edgeedge with one I(2)Cs3Bi2 square pyramid, edges with two equivalent I(3)Cs4Bi square pyramids, edges with two equivalent I(4)Cs4Bi square pyramids,  a faceface with one I(2)Cs3Bi2 square pyramid, and  a faceface with one I(4)Cs4Bi square pyramid. In the fifth I site, I(4) is bonded to one Cs(1), three equivalent Cs(2), and one Bi(1) atom to form distorted ICs4Bi square pyramids that share corners with three equivalent I(2)Cs3Bi2 square pyramids, corners with three equivalent I(4)Cs4Bi square pyramids, corners with five equivalent I(3)Cs4Bi square pyramids, edges with two equivalent I(3)Cs4Bi square pyramids, edges with two equivalent I(4)Cs4Bi square pyramids, edges with three equivalent I(2)Cs3Bi2 square pyramids, and  a faceface with one I(3)Cs4Bi square pyramid.
Cs3Bi2I9 crystallizes in the monoclinic C2/c space group. There are two inequivalent Cs sites. In the first Cs site, Cs(1) is bonded in a distorted q6 geometry to two equivalent I(3), two equivalent I(4), two equivalent I(5), and four equivalent I(2) atoms. Both Cs(1)-I(3) bond lengths are 4.34 Å. Both Cs(1)-I(4) bond lengths are 4.11 Å. Both Cs(1)-I(5) bond lengths are 4.26 Å. There are two shorter (4.23 Å) and two longer (4.30 Å) Cs(1)-I(2) bond lengths. In the second Cs site, Cs(2) is bonded in a 11-coordinate geometry to one I(2), one I(5), three equivalent I(1), three equivalent I(3), and three equivalent I(4) atoms. The Cs(2)-I(2) bond length is 4.22 Å. The Cs(2)-I(5) bond length is 4.51 Å. There are a spread of Cs(2)-I(1) bond distances ranging from 4.05-4.28 Å. There are a spread of Cs(2)-I(3) bond distances ranging from 4.14-4.38 Å. There are a spread of Cs(2)-I(4) bond distances ranging from 4.21-4.37 Å. Bi(1) is bonded to one I(1), one I(3), one I(4), one I(5), and two equivalent I(2) atoms to form face-sharing BiI6 octahedra. The Bi(1)-I(1) bond length is 2.96 Å. The Bi(1)-I(3) bond length is 2.97 Å. The Bi(1)-I(4) bond length is 2.98 Å. The Bi(1)-I(5) bond length is 3.24 Å. There is one shorter (3.22 Å) and one longer (3.26 Å) Bi(1)-I(2) bond length. There are five inequivalent I sites. In the first I site, I(5) is bonded in a 6-coordinate geometry to two equivalent Cs(1), two equivalent Cs(2), and two equivalent Bi(1) atoms. In the second I site, I(1) is bonded in a distorted rectangular see-saw-like geometry to three equivalent Cs(2) and one Bi(1) atom. In the third I site, I(2) is bonded to one Cs(2), two equivalent Cs(1), and two equivalent Bi(1) atoms to form ICs3Bi2 square pyramids that share corners with three equivalent I(4)Cs4Bi square pyramids, corners with four equivalent I(3)Cs4Bi square pyramids, corners with five equivalent I(2)Cs3Bi2 square pyramids,  an edgeedge with one I(3)Cs4Bi square pyramid, edges with three equivalent I(4)Cs4Bi square pyramids,  a faceface with one I(2)Cs3Bi2 square pyramid, and  a faceface with one I(3)Cs4Bi square pyramid. In the fourth I site, I(3) is bonded to one Cs(1), three equivalent Cs(2), and one Bi(1) atom to form distorted ICs4Bi square pyramids that share corners with three equivalent I(3)Cs4Bi square pyramids, corners with four equivalent I(2)Cs3Bi2 square pyramids, corners with five equivalent I(4)Cs4Bi square pyramids,  an edgeedge with one I(2)Cs3Bi2 square pyramid, edges with two equivalent I(3)Cs4Bi square pyramids, edges with two equivalent I(4)Cs4Bi square pyramids,  a faceface with one I(2)Cs3Bi2 square pyramid, and  a faceface with one I(4)Cs4Bi square pyramid. In the fifth I site, I(4) is bonded to one Cs(1), three equivalent Cs(2), and one Bi(1) atom to form distorted ICs4Bi square pyramids that share corners with three equivalent I(2)Cs3Bi2 square pyramids, corners with three equivalent I(4)Cs4Bi square pyramids, corners with five equivalent I(3)Cs4Bi square pyramids, edges with two equivalent I(3)Cs4Bi square pyramids, edges with two equivalent I(4)Cs4Bi square pyramids, edges with three equivalent I(2)Cs3Bi2 square pyramids, and  a faceface with one I(3)Cs4Bi square pyramid.