Doubling Polarizability Anisotropy of The Tetrahedral Motifs to Achieve Phase Matching for Melilite-Type Compound

Oxychalcogenides are extensively studied as infrared nonlinear optical (NLO) materials, as they may hybridize the advantages of oxides and chalcogenides. Here, new Eu₂Ga₂GeOS₆ (EGGOS) was designed using Eu₂Ga₂GeS₇ (EGGS) as the parent through anion partial substitution (APS) and synthesized by the facile flux assisted metal oxide-boron-chalcogen (FMOBQ) solid-state method. Compared with the {[Ga₂GeS₇]^4-}_∞ layer in EGGS, the {[Ga₂GeOS₆]^4-}_∞ layer in EGGOS has a greater distortion degree, thereby improving anisotropy to obtain greater birefringence and achieve phase matching, different from nonphase matching for EGGS. Through structural analysis, the first-principles and Gaussian polarizability anisotropy calculations reveal the mechanism of this transformation. This work provides ideas for designing new NLO materials with improved performances.