Recent progress in oxychalcogenides as IR nonlinear optical materials.

Currently, non-centrosymmetric oxychalcogenides, a class of newly developed heteroanionic compounds, have emerged as promising candidates for IR nonlinear optical (NLO) materials due to the fact that they can combine the impressive second-harmonic generation (SHG) responses of chalcogenides with the wide energy gaps of oxides. Moreover, multiple combinations of chalcogens and the oxygen element would, in principle, lead to more new frequency-doubling building units, enabling the extensive seeking and design of new NLO-active oxychalcogenides. In this Frontiers article, the recent developments of oxychalcogenides as IR-NLO candidates are summarized. These materials can be grouped into three types in terms of their structural dimensions: (i) two-dimensional layered CaZnOS, SrZn2OS2, Sr8Ga8O3S14, Sr6Cd2Sb6O7S10 and Sr4Pb1.5Sb5O5Se8; (ii) one-dimensional chain-typed AEGeOQ2 (AE = Sr and Ba; Q = S and Se); and (iii) zero-dimensional molecular Sr3Ge2O4Se3 and α-Na3PO3S. We discuss the rich coordination environment of mixed-anion frequency-doubling building units focusing on the correlations between their non-centrosymmetric structures and NLO properties, as well as their synthetic methods. Finally, the present challenges and future perspectives in this field are also proposed.