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 mixedanion SBUs of [GeOSe 3 ] and [GeO 3 S] are first designed through partial isovalent substitution of chalcogen atoms by O atoms in the classical [GeQ 4 ] (Q = S, Se) tetrahedra. On the basis of these SBUs, two new quaternary oxychalcogenides, Sr 3 Ge 2 O 4 Se 3 and SrGe 2 O 3 S 2 , are successfully synthesized. Sr 3 Ge 2 O 4 Se 3 crystallizes in the noncentrosymmetric space group R3m and possesses unique zero-dimensional [Ge 2 O 4 Se 3 ] 6− units consisting of highly distorted [GeOSe 3 ] tetrahedra and [GeO 4 ] 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 2 ). SrGe 2 O 3 S 2 belongs to the centrosymmetric space group P2 1 /c and features 2∞[Ge 2 O 3 S 2 ] 2− layers formed by the corner-shared [GeO 3 S] tetrahedra. Moreover, the large birefringence of SrGe 2 O 3 S 2 (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 [GeOSe 3 ] and [GeO 3 S] units on the moderate second harmonic generation response and large birefringence. The discovery of new mixed-anion SBUs of [GeOSe 3 ] and [GeO 3 S] will guide the exploration of new functional oxychalcogenides.

Rational design of a promising oxychalcogenide infrared nonlinear optical crystal

“…4b). [46][47][48][49][50][51][52] In order to better understand the origin of these excellent NLO properties, especially the moderate SHG response and birefringence, we further calculated the electronic structure of Sr 2 ZnSn 2 OS 6 through the rst-principles calculation.…”

First Investigation of Nonlinear Optical Oxychalcogenide with Three‐Dimensional Anionic Framework and Special Windmill‐Like Functional Motifs†

“…[ 11 ] Because of the different sizes, electronegativities, and polarizabilities between O 2– and S 2– anions, the heteroatomic coordination environments of the metal cations in oxychalcogenides are prone to forming high polarity, large anisotropy, and low symmetry. [ 12 ] Some noncentrosymmetric (NCS) oxychalcogenides demonstrate good NLO properties, such as Sr 6 Cd 2 Sb 6 O 7 S 10 , [ 13 ] SrGeOSe 2 , [ 14 ] BaGeOSe 2 , [ 15 ] Sr 3 Ge 2 O 4 Se 3 , [ 16 ] and La 3 Ga 3 Ge 2 S 3 O 10 . [ 17 ] Hitherto, all the studied NLO‐active oxychalcogenides crystallize in low‐dimensional anionic frameworks.…”

La3Ga3Ge2S3O10: An Ultraviolet Nonlinear Optical Oxysulfide Designed by Anion‐Directed Band Gap Engineering

“…They have been explored as anions for the visible or (mid‐) infrared regions, due to their high energy level p orbitals in the valence band maximum, associated narrowing of the band gap (or red‐shift) and low phonon energies in chalcogenides [27] . Mixing chalcogenide and oxide ions is a typical approach for band‐gap engineering, leading to the discovery of several phase‐matching oxychalcogenides with high SHG intensities (Table 1), [18, 28, 29] such as, A GeSe 2 O ( A =Sr, Ba) with Ge Q 2 O 2 tetrahedra (1.1–1.3×AgGaS 2 , E g ≈3.2 eV), [30, 31] SrZn 2 S 2 O with ZnS 2 O 2 tetrahedra (2×KDP, E g =3.89 eV), [32] and Sr 6 Cd 2 Sb 6 S 10 O 7 with SbS 4 O and SbS 5 pyramids (4×AgGaS 2 , E g =1.89 eV) [33] . Although strong SHG responses were observed in these oxychalcogenides, their band gaps are lower than 4 eV.…”