RbSe3B2O9(OH) and CsSe3B2O9(OH): one dimensional boroselenite-based anionic frameworks with second harmonic generation properties

Abstract

Two new alkali boroselenites RbSe₃B₂O₉(OH) and CsSe₃B₂O₉(OH) have been synthesized by traditional solid-state reactions. Single-crystal X-ray diffraction study indicated that they are isostructural and adopt a new type of structure, which crystallizes in the noncentrosymmetric space group P2₁2₁2₁. Optical diffuse reflectance spectrum studies emphasized that both are indirect optical transitions with values of 3.79 and 4.17 eV for RbSe₃B₂O₉(OH) and CsSe₃B₂O₉(OH), respectively. Optical analysis revealed a broad transparency window in the 0.3–8.5 μm region for both compounds. In addition, RbSe₃B₂O₉(OH) featured a relatively weak second-harmonic-generation response, and for CsSe₃B₂O₉(OH), the response is 0.8-times that of KH₂PO₄. Theoretical calculations of band structure, density of state, and linear and nonlinear optical properties were also performed to get insight into the relationships between electronic structures and their optical properties.