Ca2Ln(BS3)(SiS4) (Ln = La, Ce, and Gd): Mixed Metal Thioborate-Thiosilicates as Well-Performed Infrared Nonlinear Optical Materials

Abstract

The first examples of thioborate-thiosilicates, namely Ca2Ln(BS3)(SiS4) (Ln = La, Ce, and Gd), are synthesized by rationally designed high-temperature solid-state reactions. They crystalize in the polar space group P63mc and feature a novel three-dimensional crystal structure in which the discrete [BS3]3− and [SiS4]4− anionic groups are linked by Ca2+ and Ln3+ cations occupying the same atomic site. Remarkably, all three compounds show comprehensive properties required as promising infrared nonlinear optical materials, including phase-matchable strong second harmonic generation (SHG) responses at 2.05 µm (1.1–1.2 times that of AgGaS2), high laser-induced damage thresholds (7–10 times that of AgGaS2), wide light transmission range (0.45–11 µm), high thermal stabilities (>800 °C), and large calculated birefringence (0.126–0.149 @1064 nm), which justify the material design strategy of combining [BS3]3− and [SiS4]4− active units. Theoretical calculations suggest that their large SHG effects originate mainly from the synergy effects of the LnS6, BS3, and SiS4 groups. This work not only broadens the scope of research on metal chalcogenides but also provides a new synthetic route for mixed anionic thioborates.