Cs3Er2I9 crystallizes in the hexagonal P6_3/mmc space group. There are two inequivalent Cs sites. In the first Cs site, Cs(1) is bonded in a distorted q6 geometry to nine equivalent I(1) atoms. In the second Cs site, Cs(2) is bonded to six equivalent I(1) and six equivalent I(2) atoms to form CsI12 cuboctahedra that share corners with six equivalent Cs(2)I12 cuboctahedra and faces with six equivalent Er(1)I6 octahedra. Er(1) is bonded to three equivalent I(1) and three equivalent I(2) atoms to form ErI6 octahedra that share faces with three equivalent Cs(2)I12 cuboctahedra and  a faceface with one Er(1)I6 octahedra. There are two inequivalent I sites. In the first I site, I(1) is bonded in a distorted single-bond geometry to one Cs(2), three equivalent Cs(1), and one Er(1) atom. In the second I site, I(2) is bonded in a distorted rectangular see-saw-like geometry to two equivalent Cs(2) and two equivalent Er(1) atoms.
Cs3Er2I9 crystallizes in the hexagonal P6_3/mmc space group. There are two inequivalent Cs sites. In the first Cs site, Cs(1) is bonded in a distorted q6 geometry to nine equivalent I(1) atoms. There are three shorter (4.11 Å) and six longer (4.26 Å) Cs(1)-I(1) bond lengths. In the second Cs site, Cs(2) is bonded to six equivalent I(1) and six equivalent I(2) atoms to form CsI12 cuboctahedra that share corners with six equivalent Cs(2)I12 cuboctahedra and faces with six equivalent Er(1)I6 octahedra. All Cs(2)-I(1) bond lengths are 4.28 Å. All Cs(2)-I(2) bond lengths are 4.24 Å. Er(1) is bonded to three equivalent I(1) and three equivalent I(2) atoms to form ErI6 octahedra that share faces with three equivalent Cs(2)I12 cuboctahedra and  a faceface with one Er(1)I6 octahedra. All Er(1)-I(1) bond lengths are 2.90 Å. All Er(1)-I(2) bond lengths are 3.12 Å. There are two inequivalent I sites. In the first I site, I(1) is bonded in a distorted single-bond geometry to one Cs(2), three equivalent Cs(1), and one Er(1) atom. In the second I site, I(2) is bonded in a distorted rectangular see-saw-like geometry to two equivalent Cs(2) and two equivalent Er(1) atoms.