A promising lead-free fluoride carbonate SHG material designed from a theoretical perspective

In recent years, a series of fluoride carbonate crystals, as second harmonic generation (SHG) materials, have been considerably developed; however the lead in these materials is a potential threat to the environment. In this study, the structure, mechanical and optical properties of ABCO₃F (A = Rb, Cs; B = Ge, Sn) were systematically investigated to find alternatives to APbCO₃F. We found that the RbSnCO₃F crystal displays substantial stability and remarkable linear optical properties compared with those of the lead fluoride carbonate SHG materials. In particular, RbSnCO₃F exhibits unexpected anisotropy of refractive indices, a key feature to satisfy the phase matching condition in the ultraviolet spectral region. In addition, RbSnCO₃F shows a large SHG response, about 21 times that of KH₂PO₄ (KDP) with the coefficient of 8.21 pm V^-1. Our results highlight that RbSnCO₃F should be a candidate for a lead-free fluoride carbonate SHG material for achieving coherent light output in laser fields. This investigation also unravels the structure-property relationship of fluoride carbonates, contributing to explore more excellent SHG materials in future.