Revisiting the Crystal Structure of Layered Oxychalcogenides Ln₂O₂S₂ (Ln = La, Pr, and Nd)

La₂O₂S₂ was recently used as a precursor to prepare either a new metastable form of La₂O₂S by de-insertion of half of sulfur atoms of (S₂) dimers or quaternary compounds by insertion of a coinage metal (e.g., La₂O₂Cu₂S₂). A strong structural relationship exists between the polysulfide precursor and the synthesized products, which highlights the topochemical nature of these reactions. Nevertheless, the crystal structure of the precursor material is still a matter of debate. Namely, several structural models were reported so far in the literature with different space groups and/or crystal systems. All these models were built upon infinite [Ln₂O₂] slabs separated from each other by a flat sulfur layer of (S₂) dumbbells. Nevertheless, all (S₂) dimers within a given sulfur layer may rotate in phase by 90° compared to the ideal model that induces an overall atomic disorder in (S₂) dimer orientation along the stacking axis. This leads to some imbroglio and much confusion in the description of structural arrangement of Ln₂O₂S₂ materials. Herein, the crystal structures of La₂O₂S₂ and its Pr and Nd variants are revisited. We propose an alternative model that reconciles pre-existing structural descriptions of Ln₂O₂S₂ (Ln = La, Pr, and Nd) materials and highlights the strong dependency of the degree of long-range ordering of the sulfur layers on the synthesis conditions.