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 P2(1)2(1)2(00 gamma)00s or P2(1)(alpha 0 gamma)0, featuring different modulations of the [Sn2S7](infinity) 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 8xAgGaS(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 alpha-Ba2SnSe5 without modulation. Besides, despite the smaller E-g, the A(2)SnS(5) samples exhibit higher LIDTs owing to their smaller thermal expansion anisotropies (Ba2SnS5 (1.51)