Realizing Phase‐Matching of Nonlinear Optical [ABa ₃ Br ₂ ][Ga ₅ Se ₁₀ ] (A = Cs, Rb, K, Na) Through Large Anisotropic Lattice Distortion

Abstract Achieving phase‐matching (PM) behavior by enhancing the birefringence of nonlinear optical (NLO) crystals has been a formidable challenge. Herein, salt‐inclusion chalcogenides [ABa 3 Br 2 ][Ga 5 Se 10 ] (A = Cs, 1 ; Rb, 2 ; K, 3 ; Na, 4 ) that crystal in tetragonal system were synthesized and exhibit smaller c / a ratios (where a and c are the lattice constants) when compared with their Cl‐analogues, indicative of an anisotropic lattice distortion. Along the a ‐ and c ‐axes, the refractive indices show a significant divergence, corresponding to the decrease or increase in electron density. Such anisotropic behavior significantly enhances the birefringence of compounds 1 – 4 (0.044–0.066 @546 nm), realizing the transition from non‐PM (Cl‐analogues) to PM (compounds 1 – 4 ) characteristics at near 2.0 um laser. Moreover, compounds 1 – 4 represent a rare class of selenides that combine significant PM second‐harmonic generation responses (>1.0 × AgGaS 2 ) with wide optical band gaps (>3.0 eV). This work achieves the critical transition from non‐PM to PM behavior through significant anisotropic lattice distortion, thereby offering a new avenue for developing NLO materials.