Designing a Deep-UV Nonlinear Optical Fluorooxosilicophosphate

Structures composed of SiO x F 6– x ( x = 1, 2, 3, 4, 5) or SiO x F 4– x ( x = 1, 2, 3) species have thus far been observed in only a few compounds, and their functional properties are completely unknown in silicate chemistry. By introducing the least electronegative element, cesium, and the most electronegative element, fluorine, into the silicophosphate system, we successfully designed the first noncentrosymmetric fluorooxosilicophosphate with Si–F bonds, CsSiP 2 O 7 F, whose structure consists of an unprecedented SiP 2 O 10 F moiety containing hexacoordinate SiO 5 F species. The experimental results highlight CsSiP 2 O 7 F as the first fluorooxosilicophosphate deep-UV nonlinear optical (NLO) material. The first-principles calculations reveal that the SiP 2 O 10 F moiety is a new type of NLO-active unit and that both cesium and fluorine increase the deep-UV transparency of CsSiP 2 O 7 F. This work provides a new source of deep-UV NLO materials and insights into obtaining noncentrosymmetric structures that are indispensable to functional materials in nonlinear optics, piezoelectricity, ferroelectric, pyroelectricity, etc.