[Rb₄Cl][Cd₁₁In₉Se₂₆]: a salt inclusion chalcogenide with a diamond-like framework for infrared nonlinear optics

The macroscopic nonlinear optical (NLO) response of T2 supertetrahedra strongly depends on their arrangement in the crystal lattice. Without effective symmetry control, disordered spatial orientations of these building units cause mutual cancellation of their microscopic hyperpolarizabilities, resulting in substantial reduction of the macroscopic NLO coefficients. Regulation of T2 supertetrahedron polymerization with alkali and alkaline earth metals has garnered extensive attention. Herein, we apply a strategy using zero-dimensional (0D) guest species in salt-inclusion chalcogenides (SICs) to influence coherent alignment of T2 supertetrahedra in the three-dimensional (3D) host framework. By tuning the 3D structural polymerization of the host [Cd₁₁In₉Se₂₆]^3-via the 0D guest [Rb₄Cl]³⁺ polycation, we successfully synthesized the SIC [Rb₄Cl][Cd₁₁In₉Se₂₆]. Under the influence of the [Rb₄Cl]³⁺ polycation, the T2 supertetrahedral units exhibit highly uniform orientational alignment. Experimental measurements and theoretical calculations reveal a bandgap of 1.70 eV, while first-principles calculations give NLO coefficients of d₁₅ = d₃₁ = -10.845 pm V^-1, d₂₄ = d₃₂ = -9.957 pm V^-1, and d₃₃ = 14.757 pm V^-1. This work demonstrates the 0D-guest modulation of 3D-framework alignment and provides new avenues for designing high-performance infrared (IR) NLO crystals.