Ca₂ Ln(BS₃ )(SiS₄ ) (Ln = La, Ce, and Gd): Mixed Metal Thioborate-Thiosilicates as Well-Performed Infrared Nonlinear...

The first examples of thioborate-thiosilicates, namely Ca₂ Ln(BS₃ )(SiS₄ ) (Ln = La, Ce, and Gd), are synthesized by rationally designed high-temperature solid-state reactions. They crystalize in the polar space group P6₃ mc 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.