Infrared nonlinear optical polymorphs α- and β- SrCu2SnS4 exhibiting large second harmonic generation responses with requisite phase-matching behavior

α- and β-SrCu2SnS4 polymorphs, exhibiting special structural changes and excellent physicochemical performances, were successfully synthesized and characterized as promising IR NLO candidates. Note that they satisfy the essential phase-matching condition and have strong SHG responses about 0.7 and 1.0 times those of benchmark AgGaS2 under 2.09 μm incident light, respectively, which agree well with the theoretical SHG coefficients and suitable birefringences based on first-principles calculation. Moreover, it is also verified that their SHG effects originate from the synergetic contribution between CuS4 and SnS4 ligands after the SHG density calculations.