Designing a Deep-UV Nonlinear Optical Fluorooxosilicophosphate

Abstract

Structures composed of SiO_<i>x</i>F_6-<i>x</i> (<i>x</i> = 1, 2, 3, 4, 5) or SiO_<i>x</i>F_4-<i>x</i> (<i>x</i> = 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, CsSiP₂O₇F, whose structure consists of an unprecedented SiP₂O₁₀F moiety containing hexacoordinate SiO₅F species. The experimental results highlight CsSiP₂O₇F as the first fluorooxosilicophosphate deep-UV nonlinear optical (NLO) material. The first-principles calculations reveal that the SiP₂O₁₀F moiety is a new type of NLO-active unit and that both cesium and fluorine increase the deep-UV transparency of CsSiP₂O₇F. 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.