K2 PbX][Ga7 S12 ] (X=Cl, Br, I): The First Lead-Containing Cationic Moieties with Ultrahigh Second-Harmonic Generation and Band Gaps Exceeding the Criterion of 2.33eV.

Abstract

In contrast to anionic group theoryofnonlinearoptical(NLO)materials that second-harmonic generation (SHG) responses mainly originate from anionic groups, structural regulation on the cationic groups of salt-inclusionchalcogenides(SICs)is performed to make them also contribute to the NLOeffects. Herein, the stereochemically active lone-electron-pair Pb2+ cation is first introduced to the cationic groups of NLO SICs, and the resultant [K2 PbX][Ga7 S12 ] (X=Cl, Br, I) are isolated via solid-state method. The features of their three-dimensional structures comprise highly oriented [Ga7 S12 ]3- and [K2 PbX]3+ frameworks derived from AgGaS2 , which display the largest phase-matching SHG intensities (2.5-2.7×AgGaS2 @1800nm) among all SICs. Concurrently, three compounds manifest band gap values of 2.54, 2.49, and 2.41eV(exceedingthe criterion of 2.33eV),whichcan avoid two-photon absorption under the fundamental laser of 1064nm, along with the relatively low anisotropy of thermal expansion coefficients, leading to improved laser-induceddamagethresholds(LIDTs) values of 2.3, 3.8, and 4.0 times that of AgGaS2 . In addition, the density of states and SHG coefficient calculations demonstrate that the Pb2+ cations narrow the band gaps and benefit SHG responses.