Large Second-Harmonic Generation Responses Achieved by the Dimeric [Ge₂Se₄(μ-Se₂)]^4– Functional Motif in Polar Polyselenides A₄Ge₄Se₁₂ (A = Rb, Cs)

Two new polar polyselenides Rb₄Ge₄Se₁₂ (<b>1</b>) and Cs₄Ge₄Se₁₂ (<b>2</b>) with rarely reported dimeric [Ge₂Se₄(μ-Se₂)]^4– units were synthesized. They present large second-harmonic generation (SHG) intensities of 7.5 and 6.5 times that of the benchmark AgGaS₂ with type I phase-matching behavior, high laser-induced damaged thresholds, a wide transmission region and congruently melting behavior, making them excellent candidates for IR nonlinear optical (NLO) applications. The SHG functional motifs of both compounds are determined to be [Ge₂Se₄(μ-Se₂)]^4– unit by time-dependent density functional theory calculation, which further reveals that charge transfers from the lone pairs of terminal Se atoms to the five σ* orbitals of five-membered ring Ge₂Se₃ have a predominant contribution to the total SHG effect