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Researchers Design First Lead/Tin Fluorooxoborates with Widely Divergent Second Harmonic Generation Effects

Jun 21, 2018

Nonlinear optical (NLO) materials have attracted great attention as they can produce coherent light via frequency conversion in all-solid-state lasers. Due to the increasing demand for high-performance crystal, researchers are focusing on new NLO crystals with large macroscopic second harmonic generation (SHG) effects and wide transparencies.

In a study published in Journal of the American Chemical Society, the research group led by Prof. YE Ning at Fujian Institute of Research on the Structure of Matter of Chinese Academy of Sciences designed the first lead/tin fluorooxoborates MB2O3F2(M=Pb, Sn), using the nonlinear optical crystal Sr2Be2B2O7 (SBBO) as a parent model.

Compared with the rigid [Be6B6O15] double layers in SBBO, MB2O3F2 have flexible two-dimensional [B6O12F6] single layer, which not only keeps the NLO-favorable layered structure, but also overcomes the structural instability issues of SBBO.

Besides, UV/vis/NIR diffuse-reflectance spectra showed that MB2O3F2(M=Pb, Sn) exhibited relatively short UV absorption edges of ~220 and 250 nm, respectively, far shorter than those of recently reported materials containing NLO-active structural units, such as Pb3Mg3TeP2O14 (250 nm), Pb2(NO3)2(H2O)F2 (300 nm), Pb2BO3I (330 nm), Bi3TeBO9 (385 nm) and K3[V(O2)2O]CO3 (420 nm).

Interestingly, researchers found that these two fluorooxoborates showed widely divergent second harmonic responses (13×KDP for PbB2O3F2 and 4×KDP for SnB2O3F2), although they are isostructural and both contain stereochemically active lone-pair cations.

Further theoretical investigations revealed that the SHG difference is mainly attributed to the different anisotropies of Pb and Sn SHG-active orbitals, which has devastating effects on the SHG effects in PbB2O3F2 and SnB2O3F2, respectively.

This study revealed two promising UV NLO materials as well as the mechanism of stereochemically active lone-pair (SCALP) cations' contribution to hyperpolarizability.

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