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The new rubidium-containing erbium sulfide thioarsenate(III) with the structured formula RbEr₂S(S₂)[AsS₂(S₂)] was obtained from the syntheses of elemental erbium (Er), arsenic sesquisulfide (As₂S₃) and rubidium sesquisulfide (Rb₂S₃) with elemental sulfur (S) at 773 K as transparent, orange, needle-shaped crystals. RbEr₂AsS₇ crystallizes monoclinically in the space group <i>C</i>2/<i>c</i> with <i>a</i> = 2339.86(12) pm, <i>b</i> = 541.78(3) pm, <i>c</i> = 1686.71(9) pm and <i>β</i> = 93.109(3) ° for <i>Z</i> = 8. The crystal structure features complex [AsS₂(S₂)]³⁻ anions with two S²⁻ anions and a (S₂)²⁻ disulfide dumbbell coordinating end-on as ligands for each As³⁺ cation. Even outside the ligand sphere of As³⁺, S²⁻ and (S₂)²⁻ can be found as sulfide anions. Two distinct Er³⁺ cations are surrounded by either nine or seven sulfur atoms. The [ErS₉] polyhedra are corner- and face-connected, while the [ErS₇] units share common edges, both building chains along [010]. These different chains undergo edge connectivity with each other, resulting in the formation of corrugated layers, which are held together by Rb⁺ in chains of condensed [RbS₉] polyhedra. So, a three-dimensional network is generated, offering empty channels along [010] apt to take up the As³⁺ lone-pair cations. Wavelength-dispersive X-ray spectroscopy verified a molar Rb:Er:As:S ratio of approximately 1:2:1:7 and diffuse reflectance spectroscopy showed the typical <i>f</i>–<i>f</i> transitions of Er³⁺, while the optical band gap was found to be 2.42 eV.