A${}_2$SnS${}_5$. A structural incommensurate modulation exhibiting strong second-harmonic generation and a high laser-induced damage threshold (A=Ba, Sr)

Structural modulations have been recently found to cause some unusual physical properties, such as superconductivity or charge density waves; however, thus-induced nonlinear optical properties are rare. We report herein two unprecedented incommensurately modulated nonlinear optical sulfides exhibiting phase matching behavior, A${}_2$SnS${}_5$ (A=Ba, Sr), with the (3+1)D superspace groups P2${}_1$2${}_1$2(00γ)00s or P2${}_1$(α0γ)0, featuring different modulations of the [Sn${}_2$S${}_7$]${}_{\infty }$ belts. Remarkably, Ba${}_2$SnS${}_5$ exhibits an excellent second harmonic generation (SHG) of 1.1 times that of the benchmark compound AgGaS${}_2$ at 1570 nm and a very large laser-induced damage threshold (LIDT) of 8 $\times$ AgGaS${}_2$. Theoretical studies revealed that the structural modulations increase the distortions of the Sn/S building units by about 44 or 25 % in A${}_2$SnS${}_5$ (A=Ba, Sr), respectively, and enhance significantly the SHG compared with α‐Ba${}_2$SnSe${}_5$ without modulation. Besides, despite the smaller E${}_g$, the A${}_2$SnS${}_5$ samples exhibit higher LIDTs owing to their smaller thermal expansion anisotropies (Ba${}_2$SnS${}_5$ (1.51)2SnS${}_5$ (2.08)2 (2.97)). (© 2020 Wiley‐VCH Verlag GmbH and Co. KGaA, Weinheim)