A₂ SnS₅ : A Structural Incommensurate Modulation Exhibiting Strong Second-Harmonic Generation and a High...

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, A₂ SnS₅ (A=Ba, Sr), with the (3+1)D superspace groups P2₁ 2₁ 2(00γ)00s or P2₁ (α0γ)0, featuring different modulations of the [Sn₂ S₇ ]_∞ belts. Remarkably, Ba₂ SnS₅ 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 A₂ SnS₅ (A=Ba, Sr), respectively, and enhance significantly the SHG compared with α-Ba₂ SnSe₅ without modulation. Besides, despite the smaller E_g , the A₂ SnS₅ samples exhibit higher LIDTs owing to their smaller thermal expansion anisotropies (Ba₂ SnS₅  (1.51)2 SnS₅  (2.08)2  (2.97)).