Functionalization Based on the Substitutional Flexibility: Strong Middle IR Nonlinear Optical Selenides AXII4XIII5Se12

Abstract

Seven nonlinear optical (NLO) active selenides in the middle IR region, AX^II₄X^III₅Se₁₂ (A = K⁺–Cs⁺; X^II = Mn²⁺, Cd²⁺; X^III = Ga³⁺, In³⁺) adopting the KCd₄Ga₅S₁₂-type structure, have been synthesized by high-temperature solid-state reaction of an elemental mixture with ACl flux. Their three-dimensional network structures are stacked by M₉Se₂₄-layers of vertex sharing MSe₄ tetrahedra, of which each center is jointly occupied by X^II and X^III atoms. Studies suggest that such tetrahedral building units can be regarded as the “multi-functional sites”, on which the Cd²⁺/Ga³⁺ pair gives rise to the coexistence of NLO and thermochromic properties, and the Mn²⁺/In³⁺ pair leads to the coexistence of NLO and magnetic properties. The density functional theory (DFT) studies and the cutoff-energy-dependent NLO coefficient analyses reveal that such “multi-functional sites” contribute to the origin of the second harmonic generation (SHG) that is ascribed to the electronic transitions from the Se-4p states to the ns, np states of X^II and X^III atoms. Remarkably, title compounds show very strong SHG at an incident wavelength of 2.05 μm, roughly 16–40 times that of commercial AgGaS₂; among them, ACd₄In₅Se₁₂ (A = Rb, Cs) represents the strongest SHG among chalcogenides to date.