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

The first examples of thioborate-thiosilicates, namely Ca 2 Ln(BS 3 )(SiS 4 ) (Ln = La, Ce, and Gd), are synthesized by rationally designed high-temperature solid-state reactions. They crystalize in the polar space group P 6 3 mc and feature a novel three-dimensional crystal structure in which the discrete [BS 3 ] 3− and [SiS 4 ] 4− anionic groups are linked by Ca 2+ and Ln 3+ 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 AgGaS 2 ), high laser-induced damage thresholds (7–10 times that of AgGaS 2 ), 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 [BS 3 ] 3− and [SiS 4 ] 4− active units. Theoretical calculations suggest that their large SHG effects originate mainly from the synergy effects of the LnS 6 , BS 3 , and SiS 4 groups. This work not only broadens the scope of research on metal chalcogenides but also provides a new synthetic route for mixed anionic thioborates.