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.

Solvothermal and mechanochemical intercalation of Cu into La2O2S2 enabled by the redox reactivity of (S2)2- pairs.

Intercalation of Cu into layered polychalcogenide La2O2S2 was demonstrated to be viable both under solvothermal conditions at 200 °C and mechanical ball milling at ambient temperature. This result evidences the soft-chemical nature of metal intercalation into layered polychalcogenides driven by the redox reactivity of anion-anion bonds.

Design of metastable oxychalcogenide phases by topochemical (de)intercalation of sulfur in La2O2S2.

Designing and synthesising new metastable compounds is a major challenge of today's material science. While exploration of metastable oxides has seen decades-long advancement thanks to the topochemical deintercalation of oxygen as recently spotlighted with the discovery of nickelate superconductor, such unique synthetic pathway has not yet been found for chalcogenide compounds. Here we combine an original soft chemistry approach, structure prediction calculations and advanced electron microscopy techniques to demonstrate the topochemical deintercalation/reintercalation of sulfur in a layered oxychalcogenide leading to the design of novel metastable phases. We demonstrate that La2O2S2 may react with monovalent metals to produce sulfur-deintercalated metastable phases La2O2S1.5 and oA-La2O2S whose lamellar structures were predicted thanks to an evolutionary structure-prediction algorithm. This study paves the way to unexplored topochemistry of mobile chalcogen anions.

Prediction of a New Layered Polymorph of FeS2 with Fe3+S2-(S22-)1/2 Structure.

The never-elucidated crystal structure of metastable iron disulfide FeS2 resulting from the full deintercalation of Li in Li2FeS2 has been cracked thanks to crystal structure prediction searches based on an evolutionary algorithm combined with first-principles calculations accounting for experimental observations. Besides the newly layered C2/m polymorph of iron disulfide, two-dimensional dynamically stable FeS2 phases are proposed that contain sulfides and/or persulfide S2 motifs.