Deep-ultraviolet (UV) nonlinear optical materials play a vital role in a variety of high-tech scientific instruments. Traditionally, the sources of these materials were usually limited to π-conjugated systems such as borates and carbonates, while the non-π-conjugated system such as phosphates and silicates is relatively unexplored. 

In a study published in J. Am. Chem. Soc., a research group led by Prof. LUO Junhua and Prof. ZHAO Sangen from Fujian Institute of Research on the Structure of Matter (FJIRSM) of Chinese Academy of Sciences, reported a new non-π-conjugated nonlinear optical fluorooxosilicophosphate CsSiP₂O₇F, which is the first non-centrosymmetric fluorooxosilicophosphate with Si-F bonds. 

The researchers found that the introduction of the element cesium with the least electronegative property and the element fluorine with the largest electronegative helps to form the non-centrosymmetric structure of CsSiP₂O₇F. In this structure, the element cesium forms locally asymmetric CsO₅F₂ polyhedra and element fluorine forms SiO₅F species to lower the local symmetry of SiP₂O₁₀F moiety. 

The second harmonic generation (SHG) intensity of powder CsSiP₂O₇F sample is approximately 0.7 times that of KH₂PO₄ and shows phase-matching behavior. According to the first-principles calculations, the SHG response mainly results from the unprecedented SiP₂O₁₀F moiety. As a result, the SiP₂O₁₀F moiety in this structure is a new type of nonlinear optical active gene. 

In addition, relevant experimental and calculated results indicated that CsSiP₂O₇F is deep-UV transparent, and both cesium and fluorine favor the deep-UV transparency of CsSiP₂O₇F. 

This study provides a new source for deep-UV nonlinear optical materials, and insight into how to obtain non-centrosymmetric structures that are indispensible to functional materials on nonlinear optics, piezoelectricity, ferroelectric, pyroelectricity, etc. 

Schematic illustration of the research (Image by Prof. LUO’s group)