Exploring And Research Of Nonlinear Optical Crystals In Fluoride Carbonates

In this work, [CO₃^2-] groups with a planar triangle structure were selected as NLO functional structual units and the use of different cations to modify the arrangement of anionic groups to creat new noncentrosymmetry ?NCS? compounds was put forward. Based on the above structural design idea, a new series of fluoride carbonates ?KCdCO₃F, RbCdCO₃F, KZnCO₃F, and RbZnCO₃F? have been synthesized under subcritical hydrothermal technology. The structure of the four compounds have been determined by single crystal X-ray diffraction studies. And further characterizations of powder X-ray diffraction, element analysis, UV-Vis spectra and thermogravimetric analysis, have also been performed. We found that all crystals were isostructural and exhibited the stacking of alternating [AF]?A= K, Rb) and [TCO₃]??T = Zn, Cd? layers connecting adjacent layers by infinite T-F-T ?T = Zn, Cd? chains parallel to c-axis to construct 3D network architecture. Within a single [CdCO₃]? layer, all [CO₃^2-] groups were perfectly aligned in the same direction in the a-b plane, but rotated from one layer to the next as a result of the nonparallel arrangement of [CO₃^2-] groups between two adjacent [TC03]? layers. The relative rotation of [CO₃^2-] groups between two successive layers was successfully controlled by introducing cations of different sizes into the structures, which led to different relative rotation angles of [CO₃^2-] groups for each fluoride carbonates. In this paper, we explained in detail that how cations controled the rotation of [CO₃^2-] groups in ATCO₃F, and also discussed the relationship between the rotation of [CO₃^2-] groups and NLO performance based on the anionic group theory. Furthermore, Sencond-Harmonic generation ?SHG? measurements, thermal property and UV-Vis spectrum were performed. The SHG measurement indicates these compounds are all phase-matchable materials in both the visible and the UV region, and the experimental SHG responses are approximately 4.58,2.84,1.76, and 0.83 times that of KH2PO4 ?KDP? for KCdCO₃F, RbCdCO₃F, KZnCO₃F, and RbZnCO₃F, respectively. All new compounds exhibit wide transparent regions ranging from the UV to the near IR. These properties suggest that they are promising UV NLO materials. Besides, the different features of the structures of A1+M2+CO₃F-type crystals were reviewed; The orientation of [CO₃^2-] groups and the structural modulation of anionic group architectures by cations were summaried and the relationship between the arrangement of anionic group and NLO properties was revealed. Our study indicated that the different alignments among the [CO₃^2-] groups originated from the coordination behaviors of the countercations, which can be distinguished by the ratios of ionic radii between the A+ ?K, Rb, Cs? and M2+ ?Zn, Cd, Ca, Sr? cations. With the ratio range from 1.2 to 2.2 corresponding to the variable coordination number of countercations, the alignment of [CO₃^2-] groups changed from fully coparallel to coplanar but not fully coparallel. It was found that noly cations having appropriate coordination number could make the [CO₃^2-] groups completely arranged on the equatorial plane of countercations, and only when [CO₃^2-] groups coordination to countercations were all monodentate or bidentate ligands, could they aligned to the same direction, and only [CO₃^2-] groups having at least one bidentate ligands, could they not rotate in the processs of the layer stacking. Therfore, we proposed that controlling cation coordination sphere by combining the moderate size of cations affected the relative positions between cations and [CO₃^2-] groups, which could force the anionic groups to be coplanar and arranged in parallel to obtain greater SHG responses and birefringences. Their structural design methods with respect to the structural modulation of anionic group architectures by cations to optimize SHG effects might be a particularly useful approach for the design of new SHG compounds in the carbonate or borate families.