[K2PbX][Ga7S12] (X = Cl, Br, I): The First Lead-Containing Cationic Moieties with Ultrahigh Second-Harmonic Generation and Band Gaps Exceeding the Criterion of 2.33 eV

Abstract

In contrast to anionic group theory of nonlinear optical (NLO) materials that second-harmonic generation (SHG) responses mainly originate from anionic groups, structural regulation on the cationic groups of salt-inclusion chalcogenides (SICs) is performed to make them also contribute to the NLO effects. Herein, the stereochemically active lone–electron-pair Pb2+ cation is first introduced to the cationic groups of NLO SICs, and the resultant [K2PbX][Ga7S12] (X = Cl, Br, I) are isolated via solid-state method. The features of their three-dimensional structures comprise highly oriented [Ga7S12]3− and [K2PbX]3+ frameworks derived from AgGaS2, which display the largest phase-matching SHG intensities (2.5−2.7 × AgGaS2 @1800 nm) among all SICs. Concurrently, three compounds manifest band gap values of 2.54, 2.49, and 2.41 eV (exceeding the criterion of 2.33 eV), which can avoid two-photon absorption under the fundamental laser of 1064 nm, along with the relatively low anisotropy of thermal expansion coefficients, leading to improved laser-induced damage thresholds (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.