Revisiting the Crystal Structure of Layered Oxychalcogenides Ln 2 O 2 S 2 (Ln = La, Pr, and Nd).

La 2 O 2 S 2 was recently used as a precursor to prepare either a new metastable form of La 2 O 2 S by de-insertion of half of sulfur atoms of (S 2 ) dimers or quaternary compounds by insertion of a coinage metal (e.g., La 2 O 2 Cu 2 S 2 ). 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 [Ln 2 O 2 ] slabs separated from each other by a flat sulfur layer of (S 2 ) dumbbells. Nevertheless, all (S 2 ) dimers within a given sulfur layer may rotate in phase by 90° compared to the ideal model that induces an overall atomic disorder in (S 2 ) dimer orientation along the stacking axis. This leads to some imbroglio and much confusion in the description of structural arrangement of Ln 2 O 2 S 2 materials. Herein, the crystal structures of La 2 O 2 S 2 and its Pr and Nd variants are revisited. We propose an alternative model that reconciles pre-existing structural descriptions of Ln 2 O 2 S 2 (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.