Rb4CO4 crystallizes in the monoclinic Cm space group. There are three inequivalent Rb sites. In the first Rb site, Rb(1) is bonded to one O(2), two equivalent O(1), and two equivalent O(3) atoms to form distorted RbO5 trigonal bipyramids that share corners with two equivalent Rb(1)O5 trigonal bipyramids,  an edgeedge with one C(1)O4 tetrahedra, and  a faceface with one C(1)O4 tetrahedra. In the second Rb site, Rb(2) is bonded in a 4-coordinate geometry to one O(2), one O(3), and two equivalent O(1) atoms. In the third Rb site, Rb(3) is bonded in a 4-coordinate geometry to one O(2), one O(3), and two equivalent O(1) atoms. C(1) is bonded to one O(2), one O(3), and two equivalent O(1) atoms to form CO4 tetrahedra that share  an edgeedge with one Rb(1)O5 trigonal bipyramid and  a faceface with one Rb(1)O5 trigonal bipyramid. There are three inequivalent O sites. In the first O site, O(1) is bonded in a distorted single-bond geometry to one Rb(1), one Rb(2), two equivalent Rb(3), and one C(1) atom. In the second O site, O(2) is bonded in a distorted single-bond geometry to one Rb(1), one Rb(2), two equivalent Rb(3), and one C(1) atom. In the third O site, O(3) is bonded in a single-bond geometry to one Rb(2), two equivalent Rb(1), two equivalent Rb(3), and one C(1) atom.
Rb4CO4 crystallizes in the monoclinic Cm space group. There are three inequivalent Rb sites. In the first Rb site, Rb(1) is bonded to one O(2), two equivalent O(1), and two equivalent O(3) atoms to form distorted RbO5 trigonal bipyramids that share corners with two equivalent Rb(1)O5 trigonal bipyramids,  an edgeedge with one C(1)O4 tetrahedra, and  a faceface with one C(1)O4 tetrahedra. The Rb(1)-O(2) bond length is 2.96 Å. Both Rb(1)-O(1) bond lengths are 2.67 Å. There is one shorter (2.95 Å) and one longer (2.96 Å) Rb(1)-O(3) bond length. In the second Rb site, Rb(2) is bonded in a 4-coordinate geometry to one O(2), one O(3), and two equivalent O(1) atoms. The Rb(2)-O(2) bond length is 2.73 Å. The Rb(2)-O(3) bond length is 3.24 Å. Both Rb(2)-O(1) bond lengths are 2.79 Å. In the third Rb site, Rb(3) is bonded in a 4-coordinate geometry to one O(2), one O(3), and two equivalent O(1) atoms. The Rb(3)-O(2) bond length is 3.12 Å. The Rb(3)-O(3) bond length is 2.81 Å. There is one shorter (2.66 Å) and one longer (2.71 Å) Rb(3)-O(1) bond length. C(1) is bonded to one O(2), one O(3), and two equivalent O(1) atoms to form CO4 tetrahedra that share  an edgeedge with one Rb(1)O5 trigonal bipyramid and  a faceface with one Rb(1)O5 trigonal bipyramid. The C(1)-O(2) bond length is 1.44 Å. The C(1)-O(3) bond length is 1.45 Å. Both C(1)-O(1) bond lengths are 1.44 Å. There are three inequivalent O sites. In the first O site, O(1) is bonded in a distorted single-bond geometry to one Rb(1), one Rb(2), two equivalent Rb(3), and one C(1) atom. In the second O site, O(2) is bonded in a distorted single-bond geometry to one Rb(1), one Rb(2), two equivalent Rb(3), and one C(1) atom. In the third O site, O(3) is bonded in a single-bond geometry to one Rb(2), two equivalent Rb(1), two equivalent Rb(3), and one C(1) atom.