Enhanced Second-Harmonic-Generation Efficiency andBirefringence in Melillite Oxychalcogenides Sr2MGe2OS6(M = Mn,Zn, and Cd)

Simultaneous regulation and control of non-linearsecond-harmonic-generation (SHG) coefficients (deff) and linearbirefringence (Delta n) of non-centrosymmetric (NCS) crystals is acrucial aspect of improving the non-linear optical (NLO)performance, yet it remains a huge challenge in modern lasertechniques and science. Herein, a series of melilite-type oxide-derived NCS oxychalcogenides, Sr2MGe2OS6(M = Mn, Zn, andCd), are successfully designed and synthesized through a facilepartial isovalent chemical substitution strategy. These isostructuralcompounds crystallize in the tetragonal NCS space groupP4??21m(no. 113) and feature unique two-dimensional Cairo pentagonaltiling layers composed of heteroligand [GeOS3] and tetrahedral[MS4] asymmetric building units (ABUs). Compared to the parentmelilite-type oxides, the derived title compounds not only successfully realize the phase matchability transformation [i.e., from non-phase-matching (NPM) to phase-matching (PM)] but also greatly improve the IR-NLO performance (especially, simultaneouslyboostingdeffand Delta n). Sr2CdGe2OS6exhibits the best comprehensive performance among oxychalcogenides, including a wide PMcutoffedge (>525 nm), a strongdeff(0.8xAgGaS2), a giant laser-induced damage threshold (19.2xAgGaS2), a large band gap(3.62 eV), as well as a broad transmission cutoffregion (0.28-12.0 mu m). Furthermore, highly distorted [GeOS3] ABU in thismelilite-type structure is proved to be the profitable bifunctional NLO-active unit, which can contribute to the search and design ofnovel IR-NLO crystals with a large Delta nand a strongdeffbased on the experimental results and theoretical calculations. This workdemonstrates thefirst examples of PM melilite-type oxychalcogenides and offers new perspectives on the phase matchabilitytransformation through the partial isovalent chemical substitution approach, which will play a constructive role in the future designof high-performance IR-NLO heteroanionic materials.

Automated summary

This study successfully designed and synthesized a series of non-centrosymmetric oxychalcogenides through a simple chemical substitution strategy. These compounds not only achieved phase matchability transformation but also significantly improved the infrared non-linear optical performance. Among them, Sr2CdGe2OS6 exhibited the best comprehensive performance.