Cs3Tl2Cl9 crystallizes in the trigonal R-3c space group. Cs(1) is bonded to four equivalent Cl(1) and eight equivalent Cl(2) atoms to form CsCl12 cuboctahedra that share corners with six equivalent Cs(1)Cl12 cuboctahedra, corners with four equivalent Tl(1)Cl6 octahedra, faces with eight equivalent Cs(1)Cl12 cuboctahedra, and faces with four equivalent Tl(1)Cl6 octahedra. The corner-sharing octahedral tilt angles range from 11-31°. Tl(1) is bonded to three equivalent Cl(1) and three equivalent Cl(2) atoms to form distorted TlCl6 octahedra that share corners with six equivalent Cs(1)Cl12 cuboctahedra, faces with six equivalent Cs(1)Cl12 cuboctahedra, and  a faceface with one Tl(1)Cl6 octahedra. There are two inequivalent Cl sites. In the first Cl site, Cl(2) is bonded in a distorted single-bond geometry to four equivalent Cs(1) and one Tl(1) atom. In the second Cl site, Cl(1) is bonded to four equivalent Cs(1) and two equivalent Tl(1) atoms to form a mixture of distorted corner, edge, and face-sharing ClCs4Tl2 octahedra. The corner-sharing octahedral tilt angles range from 33-61°.
Cs3Tl2Cl9 crystallizes in the trigonal R-3c space group. Cs(1) is bonded to four equivalent Cl(1) and eight equivalent Cl(2) atoms to form CsCl12 cuboctahedra that share corners with six equivalent Cs(1)Cl12 cuboctahedra, corners with four equivalent Tl(1)Cl6 octahedra, faces with eight equivalent Cs(1)Cl12 cuboctahedra, and faces with four equivalent Tl(1)Cl6 octahedra. The corner-sharing octahedral tilt angles range from 11-31°. There are two shorter (3.79 Å) and two longer (3.83 Å) Cs(1)-Cl(1) bond lengths. There are a spread of Cs(1)-Cl(2) bond distances ranging from 3.66-3.79 Å. Tl(1) is bonded to three equivalent Cl(1) and three equivalent Cl(2) atoms to form distorted TlCl6 octahedra that share corners with six equivalent Cs(1)Cl12 cuboctahedra, faces with six equivalent Cs(1)Cl12 cuboctahedra, and  a faceface with one Tl(1)Cl6 octahedra. All Tl(1)-Cl(1) bond lengths are 2.75 Å. All Tl(1)-Cl(2) bond lengths are 2.51 Å. There are two inequivalent Cl sites. In the first Cl site, Cl(2) is bonded in a distorted single-bond geometry to four equivalent Cs(1) and one Tl(1) atom. In the second Cl site, Cl(1) is bonded to four equivalent Cs(1) and two equivalent Tl(1) atoms to form a mixture of distorted corner, edge, and face-sharing ClCs4Tl2 octahedra. The corner-sharing octahedral tilt angles range from 33-61°.