Cs3ErO3 crystallizes in the monoclinic C2/m space group. There are three inequivalent Cs sites. In the first Cs site, Cs(1) is bonded in a 4-coordinate geometry to four equivalent O(1) atoms. In the second Cs site, Cs(2) is bonded in a 4-coordinate geometry to four equivalent O(1) atoms. In the third Cs site, Cs(3) is bonded in a 5-coordinate geometry to one O(2) and four equivalent O(1) atoms. Er(1) is bonded to two equivalent O(1) and two equivalent O(2) atoms to form edge-sharing ErO4 tetrahedra. There are two inequivalent O sites. In the first O site, O(1) is bonded in a 7-coordinate geometry to two equivalent Cs(1), two equivalent Cs(2), two equivalent Cs(3), and one Er(1) atom. In the second O site, O(2) is bonded in a 3-coordinate geometry to one Cs(3) and two equivalent Er(1) atoms.
Cs3ErO3 crystallizes in the monoclinic C2/m space group. There are three inequivalent Cs sites. In the first Cs site, Cs(1) is bonded in a 4-coordinate geometry to four equivalent O(1) atoms. There are two shorter (2.91 Å) and two longer (3.36 Å) Cs(1)-O(1) bond lengths. In the second Cs site, Cs(2) is bonded in a 4-coordinate geometry to four equivalent O(1) atoms. There are two shorter (3.10 Å) and two longer (3.22 Å) Cs(2)-O(1) bond lengths. In the third Cs site, Cs(3) is bonded in a 5-coordinate geometry to one O(2) and four equivalent O(1) atoms. The Cs(3)-O(2) bond length is 2.82 Å. There are two shorter (3.18 Å) and two longer (3.32 Å) Cs(3)-O(1) bond lengths. Er(1) is bonded to two equivalent O(1) and two equivalent O(2) atoms to form edge-sharing ErO4 tetrahedra. Both Er(1)-O(1) bond lengths are 2.09 Å. There is one shorter (2.15 Å) and one longer (2.16 Å) Er(1)-O(2) bond length. There are two inequivalent O sites. In the first O site, O(1) is bonded in a 7-coordinate geometry to two equivalent Cs(1), two equivalent Cs(2), two equivalent Cs(3), and one Er(1) atom. In the second O site, O(2) is bonded in a 3-coordinate geometry to one Cs(3) and two equivalent Er(1) atoms.