Discovery of Noncentrosymmetric Sr 3 As 2 S 7 and Sr 3 As 2 Se 2.5 S 4.5 Featuring As 3+ /As 5+ Coordination and Their Strong Nonlinear Optical Response

Chalcoarsenates are a family of compounds that have shown promise as nonlinear optical materials. The varying oxidizing powers of chalcogens result in chalcoarsenates containing either As 3+ or As 5+, which exhibit different crystal structures. Two noncentrosymmetric chalcoarsenates, Sr 3 As 2 S 7 and Sr 3 As 2 Se 2.5 S 4.5, have been synthesized, and their structures consist of both As 3+ centered trigonal pyramids and As 5+ centered tetrahedra. These are the first reported chalcoarsenates made up of both As-units. Single-crystal X-ray diffraction reveals that they adopt their own crystal structure types. Sr 3 As 2 S 7 crystallizes in trigonal space group P 3 ( a = 17.6283 (3), c = 7.1236 (1) Å, Z = 4), and Sr 3 As 2 Se 2.5 S 4.5 crystallizes in hexagonal P 6 3 mc space group ( a = 10.1997(3) Å, c = 6.8443(4) Å, Z = 1). Consistent with their colors, diffuse reflectance measurements confirm the band gaps of 1.6 eV for Sr 3 As 2 Se 2.5 S 4.5 (dark brown) and 2.1 eV for Sr 3 As 2 S 7 (yellow). Second harmonic generation (SHG) measurements show that Sr 3 As 2 S 4.5 Se 2.5 has a strong SHG response of χ eff (2) = 90 pm/V, which is the highest among alkaline earth containing chalcoarsenates and 1.6 × AgGaSe 2 . Sr 3 As 2 S 7 shows a high laser-induced damage threshold of 2.11 GW/cm 2 . The promising SHG results highlight the need to study the Sr-containing chalcoarsenates, which are significantly underexplored.