From oxides to oxysulfides: the mixed-anion GeS(3)O unit induces huge improvement in the nonlinear optical effect and optical anisotropy for potential nonlinear optical materials

Oxysulfides combining intrinsic performance advantages between sulfides (strong NLO response) and oxides (wide optical bandgap) are proposed as potential infrared (IR) NLO materials. Theoretical calculation shows that the mixed-anion GeS(3)O tetrahedron has a stronger polarizability anisotropy and hyperpolarizability than that of the typical GeO(4) unit. Based on this, two Sr(2)MGe(2)S(6)O (M = Zn, Cd) oxysulfides with the GeS(3)O unit show dozens of times improvement in critical birefringence and the NLO effect compared with those of isostructural Sr(2)ZnGe(2)O(7). Moreover, structure–property study further verifies that the mixed-anion GeS(3)O ligand is a useful NLO-active unit and can offer great influence over the NLO origin. This research result also gives us a feasible design strategy and research system to explore new IR NLO candidates.