YouTube video by Garth Leadon (VioletRoseonceforgotten)

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 As3+ or As5+, which exhibit different crystal structures. Two noncentrosymmetric chalcoarsenates, Sr3As2S7 and Sr3As2Se2.5S4.5, have been synthesized, and their structures consist of both As3+ centered trigonal pyramids and As5+ 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. Sr3As2S7 crystallizes in trigonal space group P3 (a = 17.6283 (3), c = 7.1236 (1) Å, Z = 4), and Sr3As2Se2.5S4.5 crystallizes in hexagonal P63mc 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 Sr3As2Se2.5S4.5 (dark brown) and 2.1 eV for Sr3As2S7 (yellow). Second harmonic generation (SHG) measurements show that Sr3As2S4.5Se2.5 has a strong SHG response of χeff(2) = 90 pm/V, which is the highest among alkaline earth containing chalcoarsenates and 1.6 × AgGaSe2. Sr3As2S7 shows a high laser-induced damage threshold of 2.11 GW/cm2. The promising SHG results highlight the need to study the Sr-containing chalcoarsenates, which are significantly underexplored.

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