Nonlinear Optical Series A(VO2)3(SeO3)2 (A = NH4+, K+, and H3O+) with Large Birefringence Derived from High-Anisotropic VO6 Octahedra and SeO3 Pyramids

With the increasing significance of the laser industry, especially in the 3–5 μm range, mid-infrared (mid-IR) nonlinear optical (NLO) crystals have become crucial components to meet stringent requirements. Herein, a large birefringence is essential for a mid-IR NLO crystal, as it ensures the phase-matching condition for high-conversion efficiency in laser output operation. In this study, three vanadium selenites A(VO₂)₃(SeO₃)₂ (A = NH₄⁺, K⁺, and H₃O⁺) with hexagonal tungsten oxide-related structures were prepared, featuring two-dimensional layered structures composed of two second-order Jahn–Teller (SOJT) motifs: distorted VO₆ octahedra and SeO₃ pyramids. Owing to the highly anisotropic VO₆ octahedra and SeO₃ pyramids, the family of A(VO₂)₃(SeO₃)₂ (A = NH₄⁺, K⁺, and H₃O⁺) crystals shows significant birefringence, which represents the highest birefringence value among vanadium selenites, even surpassing some SOJT-cation-based oxides. In addition, the title crystals exhibit large second harmonic generation responses of powder of 0.28–0.58 × AgGaS₂ (AGS) at 2.09 μm.