A Surprising Continuation of the Nonisotypic Rb ₃ Ln [AsS ₄ ] ₂ Series: The Diverse Crystal Structures of the End‐Members with Ln  = Gd–Ho

The nonisotypic series of Rb 3 Ln [AsS 4 ] 2 compounds could be extended by four new monoclinic compounds, namely Rb 3 Gd[AsS 4 ] 2 , Rb 3 Tb[AsS 4 ] 2 , Rb 3 Dy[AsS 4 ] 2 , and Rb 3 Ho[AsS 4 ] 2 . Rb 3 Gd[AsS 4 ] 2 and Rb 3 Ho[AsS 4 ] 2 crystallize isopointal in the centrosymmetric space group P 2 1 / c with lattice parameters a = 962–951 pm, b = 685–681 pm, c = 2366–2392 pm, and β ≈ 91° for Z = 4. Their crystal structure shows chains of condensed [ Ln S 7 ] 11– polyhedra in a zigzag‐like fashion, propagating along [010] and separated by Rb + cations in 9‐, 10‐, and 11‐fold sulfur coordination. The here presented Rb 3 Gd[AsS 4 ] 2 is the second polymorph with this composition, accessible by increasing the synthesis temperature by 50 K up to 823 K. Rb 3 Tb[AsS 4 ] 2 and Rb 3 Dy[AsS 4 ] 2 crystallize both in the space group C 2/ c with a = 1055–1057 pm, b = 1883–1881 pm, c = 929–927 pm, and β ≈ 118° for Z = 4. Instead of zigzag chains, their structure shows linear strands of condensed [ Ln S 8 ] 13– polyhedra, again separated by Rb + cations for charge neutrality. Their coordination numbers reduce also to only eight in turn. However, for Ln = Tb and Dy, the coordination number of the Ln 3+ cations surprisingly increases from C.N. = 7 to C.N. = 8, but returns to seven immediately in the case of Ln = Ho afterward, which comes unexpectedly in terms of the lanthanoid contraction.