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, A₂SnS₅ (A=Ba, Sr), with the (3+1)D superspace groups P2₁2₁2(00γ)00s or P2₁(α0γ)0, featuring different modulations of the Sn₂S₇]_∞ belts. Remarkably, Ba₂SnS₅ exhibits an excellent second harmonic generation (SHG) of 1.1 times that of the benchmark compound AgGaS₂ at 1570 nm and a very large laser‐induced damage threshold (LIDT) of 8×AgGaS₂. Theoretical studies revealed that the structural modulations increase the distortions of the Sn/S building units by about 44 or 25 % in A₂SnS₅ (A=Ba, Sr), respectively, and enhance significantly the SHG compared with α‐Ba₂SnSe₅ without modulation. Besides, despite the smaller E_g, the A₂SnS₅ samples exhibit higher LIDTs owing to their smaller thermal expansion anisotropies (Ba₂SnS₅ (1.51)<Sr₂SnS₅ (2.08)<AgGaS₂ (2.97)).