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 (1) and Cs4Ge4Se12 (2) 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, 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.