[Rb₃BaCl][In₈Se₁₄]: Compressed chalcopyrite-type selenide achieved by polycationic substitution strategy toward excellent nonlinear optical property

Incorporation of monovalent Ag⁺ or Li⁺ ions into the anionic groups MQ₂²⁻ (M = Ga, In; Q = S, Se) within diamond-like structures holds great promise for the second-order nonlinear optical (NLO) crystals. However, these crystals are significantly challenged by a low laser-induced damage threshold (LIDT) or the drawbacks associated with silica tube corrosiveness, which severely limit their suitability for high-power applications. In this study, we employed the unusual polycationic substitution strategy based on ZnSe, specifically [Se₄Zn₁₃]Se₂₄ → [ClRb₃Ba]Se₂₄, to design and synthesize a novel salt-inclusion chalcogenide, denoted as [Rb₃BaCl][In₈Se₁₄] (1). Compound 1 shares similarity with AgInSe₂, as it maintains crystallographic symmetry in the space group I-42d and possesses a compressed chalcopyrite-type selenide structure. Remarkably, compound 1 displays remarkable phase-matching second-harmonic generation (SHG) intensity (2.0 × AgGaS₂@2.90 µm) owing to the parallel arrangement of tetrahedral InSe₄ units. Moreover, the incorporation of the polycation [Rb₃BaCl]⁴⁺ that promotes the enhancement of the band gap (2.02 eV), avoiding two-photon absorption of 2.09-µm laser, combined with a low thermal expansion coefficient, enables compound 1 to exhibit a substantial LIDT (3.2 × AgGaS₂@2.09 µm). This study establishes the potential of polycationic substitution within diamond-like structures for the rational design of exceptional mid- and far-infrared NLO materials.