2020同步年報

Neutron Science 075 Local Structural Distortion as a New Design Strategy for High-Performance Nonlinear Optical Materials A new tungsten bronze compound synthesized with a new molecular design strategy exhibits a strong SHG response, 39 times that of KDP. N onlinear optical (NLO) materials play a crucial role in modern laser technology by effectively expanding the spectral range of a laser; they have many important applications in photolithography, spectral analysis, tissue imaging, environmental monitoring, etc . Up to now, the strategies for molecular design of NLO materials have focused on the introduction of NLO-active molecular units with a large polarization, e.g ., second-order Jahn-Teller distorted octahedra (Ti 4+ , Nb 5+ , Ta 5+ , Te 6+ , W 6+ , Mo 6+ , etc .), stereochemically active lone-pair cations (Pb 2+ , Bi 3+ , I 5+ , Te 4+ , etc .), π-orbital anionic groups (BO 3 , B 3 O 6 , CO 3 , NO 3 , etc .), and d 10 metal cations with a polar displacement, into non-centrosymmetric crystal structures. 1 Kun Lin (Uni- versity of Science and Technology Beijing, China) and his coworkers recently proposed that a local structural distortion induced with vacancies, apart from the NLO-active units, can also be employed to improve the NLO effect in solids. Accordingly, a new tungsten bronze (TB) oxide, Pb 2 (Pb 0.15 Li 0.7 □ 0.15 ) Nb 5 O 15 , PLN ( □ representing vacancies), has been successfully designed and prepared, which exhibits a strong second-harmonic generation (SHG) response, 39 times that of KH 2 PO 4 (KDP)( Figs. 1(a) and 1(b) ). Multiple experimental probes were employed to study both average and lo- cal structures of PLN. To study the local structural distortion in PLN, Lin and his coworkers applied an atomic-level scanning transmission electron micro- scope (STEM) and neutron total scattering (Spallation Neutron Source, Oak Fig. 1 : (a) Experimental oscilloscope traces of SHG signals using the method of Kurtz and Perry with a 1064-nm Q-switched laser. (b) Compar- ison of powder SHG response. [Reproduced from Ref. 1] Fig. 2 : SHG response versus geometric local distortion index m for a series of lead-niobate-based TB oxides. PL06N: Pb 2.15 L i0.6 Nb 5 O 15 , PLN-Re: Pb2Li 0.94 Re 0.02 Nb 5 O 15 (Re = Eu, Gd), PKLN: Pb 2 K 0.5 Li 0.5 Nb 5 O 15 , PKN: Pb 2 KNb 5 O 15 , PNN: Pb 2 NaNb 5 O 15 , PRN: Pb 2 RbNb 5 O 15 . The dashed line serves for visual guidance; the dashed circle indicates the A-site with vacancies. [Reproduced from Ref. 1] Ridge National Laboratory). The obtained local structural information was compared with the averaged structure deduced from single-crystal X-ray diffraction, synchrotron radiation X-ray powder diffraction (BL44B2, SPring-8) and high-resolution neutron powder diffraction (BT-1, National Institute of Standards, and ECHIDNA at Australian Nuclear Science and Technology Organisation (ANSTO)). The struc- tural model of PLN, obtained from NPD and XRD, exhibits a strong site preference of the vacancy. The vacancies are distributed in quadrangular A1 tunnels surrounded by four columns of corner-sharing NbO 6 octahedra, of which the Pb sites in A2 tunnels are fully occupied. A semi-quantitative analysis of the TEM data led to the same conclusion. Lin and coworkers also observed that, in the nPDF, the signal asso- ciated with the Nb−O bond length inside the NbO 6 octahe- dra in PLN is significantly broadened. This effect indicates that the second-order Jahn-Teller-distorted NbO 6 octahedra have a larger distortion on a local scale, confirming the observations from the STEM. To demonstrate further the feasibility of the proposed design strategy, Lin and his coworkers introduced the local structural distortions on cross-substituting the A-site atoms in the TB structure and discovered a series of new compounds