CaBi4O9 crystallizes in the monoclinic P2/c space group. Ca(1) is bonded in a distorted body-centered cubic geometry to two equivalent O(2), two equivalent O(3), two equivalent O(4), and two equivalent O(5) atoms. There are two inequivalent Bi sites. In the first Bi site, Bi(1) is bonded to one O(1), one O(2), one O(3), one O(4), and two equivalent O(5) atoms to form a mixture of distorted corner and edge-sharing BiO6 octahedra. The corner-sharing octahedral tilt angles are 40°. In the second Bi site, Bi(2) is bonded in a 7-coordinate geometry to one O(1), one O(2), one O(5), two equivalent O(3), and two equivalent O(4) atoms. There are five inequivalent O sites. In the first O site, O(1) is bonded to two equivalent Bi(1) and two equivalent Bi(2) atoms to form a mixture of distorted corner and edge-sharing OBi4 trigonal pyramids. In the second O site, O(2) is bonded in a distorted trigonal non-coplanar geometry to one Ca(1), one Bi(1), and one Bi(2) atom. In the third O site, O(3) is bonded to one Ca(1), one Bi(1), and two equivalent Bi(2) atoms to form distorted OCaBi3 tetrahedra that share  a cornercorner with one O(3)CaBi3 tetrahedra, corners with three equivalent O(4)CaBi3 tetrahedra, corners with three equivalent O(1)Bi4 trigonal pyramids, corners with six equivalent O(5)CaBi3 trigonal pyramids,  an edgeedge with one O(3)CaBi3 tetrahedra, and edges with two equivalent O(4)CaBi3 tetrahedra. In the fourth O site, O(4) is bonded to one Ca(1), one Bi(1), and two equivalent Bi(2) atoms to form OCaBi3 tetrahedra that share  a cornercorner with one O(4)CaBi3 tetrahedra, corners with three equivalent O(3)CaBi3 tetrahedra,  a cornercorner with one O(1)Bi4 trigonal pyramid, corners with two equivalent O(5)CaBi3 trigonal pyramids,  an edgeedge with one O(4)CaBi3 tetrahedra, edges with two equivalent O(3)CaBi3 tetrahedra,  an edgeedge with one O(1)Bi4 trigonal pyramid, and edges with two equivalent O(5)CaBi3 trigonal pyramids. In the fifth O site, O(5) is bonded to one Ca(1), one Bi(2), and two equivalent Bi(1) atoms to form distorted OCaBi3 trigonal pyramids that share corners with two equivalent O(4)CaBi3 tetrahedra, corners with six equivalent O(3)CaBi3 tetrahedra,  a cornercorner with one O(1)Bi4 trigonal pyramid,  a cornercorner with one O(5)CaBi3 trigonal pyramid, edges with two equivalent O(4)CaBi3 tetrahedra,  an edgeedge with one O(1)Bi4 trigonal pyramid, and  an edgeedge with one O(5)CaBi3 trigonal pyramid.
CaBi4O9 crystallizes in the monoclinic P2/c space group. Ca(1) is bonded in a distorted body-centered cubic geometry to two equivalent O(2), two equivalent O(3), two equivalent O(4), and two equivalent O(5) atoms. Both Ca(1)-O(2) bond lengths are 2.35 Å. Both Ca(1)-O(3) bond lengths are 2.55 Å. Both Ca(1)-O(4) bond lengths are 2.40 Å. Both Ca(1)-O(5) bond lengths are 2.71 Å. There are two inequivalent Bi sites. In the first Bi site, Bi(1) is bonded to one O(1), one O(2), one O(3), one O(4), and two equivalent O(5) atoms to form a mixture of distorted corner and edge-sharing BiO6 octahedra. The corner-sharing octahedral tilt angles are 40°. The Bi(1)-O(1) bond length is 2.15 Å. The Bi(1)-O(2) bond length is 2.03 Å. The Bi(1)-O(3) bond length is 2.11 Å. The Bi(1)-O(4) bond length is 2.16 Å. There is one shorter (2.14 Å) and one longer (2.26 Å) Bi(1)-O(5) bond length. In the second Bi site, Bi(2) is bonded in a 7-coordinate geometry to one O(1), one O(2), one O(5), two equivalent O(3), and two equivalent O(4) atoms. The Bi(2)-O(1) bond length is 2.64 Å. The Bi(2)-O(2) bond length is 2.45 Å. The Bi(2)-O(5) bond length is 2.29 Å. There is one shorter (2.25 Å) and one longer (2.69 Å) Bi(2)-O(3) bond length. There is one shorter (2.28 Å) and one longer (2.46 Å) Bi(2)-O(4) bond length. There are five inequivalent O sites. In the first O site, O(1) is bonded to two equivalent Bi(1) and two equivalent Bi(2) atoms to form a mixture of distorted corner and edge-sharing OBi4 trigonal pyramids. In the second O site, O(2) is bonded in a distorted trigonal non-coplanar geometry to one Ca(1), one Bi(1), and one Bi(2) atom. In the third O site, O(3) is bonded to one Ca(1), one Bi(1), and two equivalent Bi(2) atoms to form distorted OCaBi3 tetrahedra that share  a cornercorner with one O(3)CaBi3 tetrahedra, corners with three equivalent O(4)CaBi3 tetrahedra, corners with three equivalent O(1)Bi4 trigonal pyramids, corners with six equivalent O(5)CaBi3 trigonal pyramids,  an edgeedge with one O(3)CaBi3 tetrahedra, and edges with two equivalent O(4)CaBi3 tetrahedra. In the fourth O site, O(4) is bonded to one Ca(1), one Bi(1), and two equivalent Bi(2) atoms to form OCaBi3 tetrahedra that share  a cornercorner with one O(4)CaBi3 tetrahedra, corners with three equivalent O(3)CaBi3 tetrahedra,  a cornercorner with one O(1)Bi4 trigonal pyramid, corners with two equivalent O(5)CaBi3 trigonal pyramids,  an edgeedge with one O(4)CaBi3 tetrahedra, edges with two equivalent O(3)CaBi3 tetrahedra,  an edgeedge with one O(1)Bi4 trigonal pyramid, and edges with two equivalent O(5)CaBi3 trigonal pyramids. In the fifth O site, O(5) is bonded to one Ca(1), one Bi(2), and two equivalent Bi(1) atoms to form distorted OCaBi3 trigonal pyramids that share corners with two equivalent O(4)CaBi3 tetrahedra, corners with six equivalent O(3)CaBi3 tetrahedra,  a cornercorner with one O(1)Bi4 trigonal pyramid,  a cornercorner with one O(5)CaBi3 trigonal pyramid, edges with two equivalent O(4)CaBi3 tetrahedra,  an edgeedge with one O(1)Bi4 trigonal pyramid, and  an edgeedge with one O(5)CaBi3 trigonal pyramid.