Structures
composed of SiO
x
F6–x
(x = 1, 2, 3, 4, 5) or SiO
x
F4–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, 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.