ASb₅S₈ (A = K, Rb, and Cs): Thermal Switching of Infrared Nonlinear Optical Properties across the Crystal/Glass Transformation

Second-order nonlinear optical (NLO) switchable materials are eliciting increased interest because of their widespread optical applications. The stringent requirements of NLO efficiency, laser-induced damage threshold (LIDT), switching ratio, and reversibility for good switches make NLO switching materials considerably rare. In this work, three ternary thioantimonate switches, ASb5S8 [A = K (1), Rb (2), and Cs (3)], with thermally driven crystal–glass transitions were obtained by flux solid-state reactions. Their structures are constructed using SbSn (n = 3–5) units with multiple coordination modes, which extend to 1-D chains and 2-D layers, and changeable alkali metals. Compounds 1 and 2 are noncentrosymmetric and present excellent infrared (IR) NLO properties with the largest phase-matchable NLO response (1.7× and 1.1× AgGaS2) among all known NLO switching materials, which primarily originate from Sb3+ with a lone electron pair and S atoms and high LIDTs (10.7× and 7.1× AgGaS2). Remarkably, compounds 1 and 2 also exhibit prominent switching characteristics, including good reversibility, moderate conductivity, and high second-harmonic generation switching ratios (365.6 and 664.5). All these features indicate their potential in practical switching applications. This work sheds lights on designing IR NLO switches by introducing lone electron pair cations with flexible coordination modes into a chalcogenide system.