Nonlinear Optical Effects Modulated by Distinctive Monovalent Cations with Lone Electron Pairs in AGaI 4 (A = Ga + , In + )

Tetrahedral halides are promising candidates for infrared (IR) nonlinear optical (NLO) materials, exhibiting balanced properties due to their wide IR transparency, large energy bandgaps, and strong NLO effects. When containing monovalent Ga⁺/In⁺ cations with lone-electron pairs, tetrahedral halides are not only capable of producing strong anisotropic optical effects, but also of maintaining an excellent performance balance. AGa₂Cl₇ (A = Ga⁺, In⁺) is an example of this distinctive system, although it has a narrower IR transparency range than AgGaS₂ and tends to deliquesce. If the polar [Ga₂Cl₇] dimer can be replaced by nonpolar [GaI₄], the resulting structure will exhibit a wider IR transparency and higher stability. In this study, GaGaI₄ is experimentally synthesized and InGaI₄ is theoretically designed with [GaI₄] to explore their potential for fulfilling this purpose. This experimental and theoretical study confirms that the existing GaGaI₄ exhibits a small powder second-harmonic signal due to counteracting [GaI₄] arrangement, and the proposed InGaI₄ can boost a stronger effect (χ₁₁₁ ≈20 pm V⁻¹) by cationic modulation and polarity optimization. Further, the Ga⁺-In⁺ system can induce a large nonlinear bulk photovoltaic effect. The findings provide insights for exploring novel IR NLO systems with distinctive properties modulated by monovalent Ga⁺/In⁺ cations.