Cs3(SeO4)2 crystallizes in the monoclinic C2/m space group. There are two inequivalent Cs sites. In the first Cs site, Cs(1) is bonded in a 10-coordinate geometry to two equivalent O(3), four equivalent O(1), and four equivalent O(2) atoms. In the second Cs site, Cs(2) is bonded in a 10-coordinate geometry to one O(2), three equivalent O(3), and six equivalent O(1) atoms. Se(1) is bonded in a tetrahedral geometry to one O(2), one O(3), and two equivalent O(1) atoms. There are three inequivalent O sites. In the first O site, O(1) is bonded in a distorted single-bond geometry to one Cs(1), three equivalent Cs(2), and one Se(1) atom. In the second O site, O(2) is bonded in a distorted single-bond geometry to one Cs(2), two equivalent Cs(1), and one Se(1) atom. In the third O site, O(3) is bonded in a distorted single-bond geometry to one Cs(1), three equivalent Cs(2), and one Se(1) atom.
Cs3(SeO4)2 crystallizes in the monoclinic C2/m space group. There are two inequivalent Cs sites. In the first Cs site, Cs(1) is bonded in a 10-coordinate geometry to two equivalent O(3), four equivalent O(1), and four equivalent O(2) atoms. Both Cs(1)-O(3) bond lengths are 3.13 Å. All Cs(1)-O(1) bond lengths are 3.08 Å. All Cs(1)-O(2) bond lengths are 3.40 Å. In the second Cs site, Cs(2) is bonded in a 10-coordinate geometry to one O(2), three equivalent O(3), and six equivalent O(1) atoms. The Cs(2)-O(2) bond length is 3.22 Å. There are two shorter (3.36 Å) and one longer (3.42 Å) Cs(2)-O(3) bond length. There are a spread of Cs(2)-O(1) bond distances ranging from 3.14-3.23 Å. Se(1) is bonded in a tetrahedral geometry to one O(2), one O(3), and two equivalent O(1) atoms. The Se(1)-O(2) bond length is 1.71 Å. The Se(1)-O(3) bond length is 1.65 Å. Both Se(1)-O(1) bond lengths are 1.64 Å. There are three inequivalent O sites. In the first O site, O(1) is bonded in a distorted single-bond geometry to one Cs(1), three equivalent Cs(2), and one Se(1) atom. In the second O site, O(2) is bonded in a distorted single-bond geometry to one Cs(2), two equivalent Cs(1), and one Se(1) atom. In the third O site, O(3) is bonded in a distorted single-bond geometry to one Cs(1), three equivalent Cs(2), and one Se(1) atom.