CsErTa6Cl18 crystallizes in the trigonal P-31c space group. Cs(1) is bonded in a 12-coordinate geometry to six equivalent Cl(1) and six equivalent Cl(3) atoms. Er(1) is bonded to six equivalent Cl(3) atoms to form ErCl6 octahedra that share corners with six equivalent Ta(1)Cl5 square pyramids. Ta(1) is bonded to one Cl(3), two equivalent Cl(1), and two equivalent Cl(2) atoms to form distorted TaCl5 square pyramids that share  a cornercorner with one Er(1)Cl6 octahedra and corners with four equivalent Ta(1)Cl5 square pyramids. The corner-sharing octahedral tilt angles are 43°. There are three inequivalent Cl sites. In the first Cl site, Cl(1) is bonded in a 2-coordinate geometry to one Cs(1) and two equivalent Ta(1) atoms. In the second Cl site, Cl(2) is bonded in a 2-coordinate geometry to two equivalent Ta(1) atoms. In the third Cl site, Cl(3) is bonded in a 2-coordinate geometry to one Cs(1), one Er(1), and one Ta(1) atom.
CsErTa6Cl18 crystallizes in the trigonal P-31c space group. Cs(1) is bonded in a 12-coordinate geometry to six equivalent Cl(1) and six equivalent Cl(3) atoms. All Cs(1)-Cl(1) bond lengths are 3.92 Å. All Cs(1)-Cl(3) bond lengths are 3.90 Å. Er(1) is bonded to six equivalent Cl(3) atoms to form ErCl6 octahedra that share corners with six equivalent Ta(1)Cl5 square pyramids. All Er(1)-Cl(3) bond lengths are 2.66 Å. Ta(1) is bonded to one Cl(3), two equivalent Cl(1), and two equivalent Cl(2) atoms to form distorted TaCl5 square pyramids that share  a cornercorner with one Er(1)Cl6 octahedra and corners with four equivalent Ta(1)Cl5 square pyramids. The corner-sharing octahedral tilt angles are 43°. The Ta(1)-Cl(3) bond length is 2.74 Å. There is one shorter (2.46 Å) and one longer (2.47 Å) Ta(1)-Cl(1) bond length. Both Ta(1)-Cl(2) bond lengths are 2.47 Å. There are three inequivalent Cl sites. In the first Cl site, Cl(1) is bonded in a 2-coordinate geometry to one Cs(1) and two equivalent Ta(1) atoms. In the second Cl site, Cl(2) is bonded in a 2-coordinate geometry to two equivalent Ta(1) atoms. In the third Cl site, Cl(3) is bonded in a 2-coordinate geometry to one Cs(1), one Er(1), and one Ta(1) atom.