From thiophosphate to chalcohalide: mixed-anion AgSxCly ligands concurrently enhancing nonlinear optical effects and laser-damage threshold

From Orthorhombic Cu8GeS6 to Cubic Cu6HgGeS6: Second-Harmonic Generation and Photocurrent Responses

A new noncentrosymmetric sulfide, Cu6HgGeS6 (CHGS), has been successfully synthesized via a heterovalent substitution strategy based on argyrodite-type Cu8GeS6 (CGS). Benefiting from the substitution of Cu+ with Hg2+ cation, the structure changes from orthorhombic Pmn21 to cubic P213. CHGS exhibits obvious nonlinear optical (NLO) activity, while CGS not. The combination of theoretical calculation and structure analysis indicates that the introduction of Hg2+ ions leads to a structural transformation, in which the trigonal pyramid [CuS3] units provide the main contribution to the NLO performance to a certain extent. Additionally, CHGS exhibits a relatively large photocurrent response (2.5 μA/cm2) among chalcogenides. This case of heterovalent substitution induced structural transformation and enhanced NLO properties provides a good example to design new NLO materials.

Nonlinear Optical Effects Modulated by Distinctive Monovalent Cations with Lone Electron Pairs in AGaI4 (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.

M6PS5X (M = Ag, Cu; X = Cl, Br) chalcohalides exhibiting strong nonlinear optical responses and high laser damage resistances

A series of M₆PS₅X (M = Ag, Cu; X = Cl, Br) nonlinear optical (NLO) chalcohalides with special MS₃X ligands have been synthesized in this work. Their critical optical performances were systematically measured and the research results show that all of them exhibit strong NLO responses (2.0-2.7 × commercial AgGaS₂) and high laser-damage thresholds (1.7-2.3 × AgGaS₂), indicating their potential application as good NLO candidates. Furthermore, first-principles calculations were used to study their inherent structure-property relationships and chalcohalides can be expected to be optimal systems for the exploration of new promising IR NLO crystals.