Two Mixed-Anion Units of [GeOSe] and [GeOS] Originating from Partial Isovalent Anion Substitution and Inducing Moderate Second Harmonic Generation Response and Large Birefringence.

Highly polarizable mixed-anion structural building units (SBUs) have been demonstrated as promising candidates for high-performing optical crystals. In this work, two new mixed-anion SBUs of [GeOSe] and [GeOS] are first designed through partial isovalent substitution of chalcogen atoms by O atoms in the classical [GeQ] (Q = S, Se) tetrahedra. On the basis of these SBUs, two new quaternary oxychalcogenides, SrGeOSe and SrGeOS, are successfully synthesized. SrGeOSe crystallizes in the noncentrosymmetric space group 3 and possesses unique zero-dimensional [GeOSe] units consisting of highly distorted [GeOSe] tetrahedra and [GeO] tetrahedra through a shared O atom. It displays intriguing potential as an infrared nonlinear optical material with a wide band gap (2.96 eV) and moderate second harmonic generation intensity (0.8 × AgGaS). SrGeOS belongs to the centrosymmetric space group 2/ and features 2∞[GeOS] layers formed by the corner-shared [GeOS] tetrahedra. Moreover, the large birefringence of SrGeOS (calculated Δ = 0.22-0.17 from 0.4 to 4.0 μm) gives it a potential as a birefringent material. Theoretical calculations revealed the crucial effects of mixed-anion [GeOSe] and [GeOS] units on the moderate second harmonic generation response and large birefringence. The discovery of new mixed-anion SBUs of [GeOSe] and [GeOS] will guide the exploration of new functional oxychalcogenides.