A₂SnS₅: 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 P 2 1 2 1 2(00 γ )00 s or P 2 1 ( α 0 γ )0, featuring different modulations of the [Sn 2 S 7 ] ∞ 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×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)<Sr 2 SnS 5 (2.08)<AgGaS 2 (2.97)).