Two Mixed-Anion Units of [GeOSe<sub>3</sub>] and [GeO<sub>3</sub>S] Originating from Partial Isovalent Anion Substitution and Inducing Moderate Second Harmonic Generation Response and Large Birefringence
Wenhao Xing, Pan Fang, Naizheng Wang, Zhuang Li, Zheshuai Lin, Jiyong Yao, Wenlong Yin, Bin Kang
Abstract
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 [GeOSe3] and [GeO3S] are first designed through partial isovalent substitution of chalcogen atoms by O atoms in the classical [GeQ4] (Q = S, Se) tetrahedra. On the basis of these SBUs, two new quaternary oxychalcogenides, Sr3Ge2O4Se3 and SrGe2O3S2, are successfully synthesized. Sr3Ge2O4Se3 crystallizes in the noncentrosymmetric space group R3m and possesses unique zero-dimensional [Ge2O4Se3]6– units consisting of highly distorted [GeOSe3] tetrahedra and [GeO4] 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 × AgGaS2). SrGe2O3S2 belongs to the centrosymmetric space group P21/c and features 2∞[Ge2O3S2]2– layers formed by the corner-shared [GeO3S] tetrahedra. Moreover, the large birefringence of SrGe2O3S2 (calculated Δn = 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 [GeOSe3] and [GeO3S] units on the moderate second harmonic generation response and large birefringence. The discovery of new mixed-anion SBUs of [GeOSe3] and [GeO3S] will guide the exploration of new functional oxychalcogenides.