Two Mixed-Anion Units of [GeOSe₃] and [GeO₃S] Originating from Partial Isovalent Anion Substitution and Inducing Moderate Second Harmonic Generation Response and Large Birefringence

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 <i>R</i>3<i>m</i> 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 <i>P</i>2₁/<i>c</i> and features 2∞[Ge₂O₃S₂]^2- layers formed by the corner-shared [GeO₃S] tetrahedra. Moreover, the large birefringence of SrGe₂O₃S₂ (calculated Δ<i>n</i> = 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.