Synthesis of Non-centrosymmetric, Metastable Rare-Earth Oxysulfides by Anionic Redox Topochemistry

Intercalation/deintercalation reactions enable introduction/removal of intercalants without destructive structure transformation of host lattices. They are among the most versatile ways to design metastable phases attainable in mild synthesis conditions. Recently, topochemical deintercalation of oxygen anions has opened up an avenue to access new compounds with unusual transition metal oxidation states and interesting properties. So far, the scope of such anion deintercalation was mainly restricted to oxides and their oxyhalide derivatives. However, lately, we presented a proof-of-concept study on the sulfur deintercalation reaction driven by anionic redox in La2O2S2. Here, we extend this work and present the design of new members of a family of slightly colored non-centrosymmetric metastable oxysulfides. Our work shows that the reduction with an alkali metal of sulfur dimers in Ln2O2S2 (Ln = Pr, Nd) precursors leads to the topochemical deinsertion of half of the sulfur atoms of each S2 pair, producing two new metastable oA-Ln2O2S (Ln = Pr, Nd) phases. The non-centrosymmetric compounds oA-Ln2O2S (Ln = La, Pr, Nd) evidence second and third harmonic generation effects, suggesting the potential applicability of the topochemical route for design of nonlinear optical materials