| Infrared(IR)second-order nonlinear optical(NLO)crystals with non-centrosymmetric(NCS)structure have important applications in military and civil fields such as mid-infrared laser generation,optoelectronic countermeasures,and infrared detection.However,the currently commercialized infrared band nonlinear optical crystals Ag Ga S2,Ag Ga Se2 and Zn Ge P2 have shortcomings such as low laser damage threshold and strong two-photon absorption,which greatly limit the development of high-power mid-and far-infrared lasers.Therefore,it is of great significance to explore new infrared nonlinear optical crystals with excellent comprehensive properties.In general,it is a great challenge to achieve the performance coexistence of large optical bandgap and strong nonlinear optical effects.In this thesis,chalcogenides are used as the research object to synthesize infrared second-order nonlinear optical crystals with NCS structures through high-temperature solid-state reaction.The crystal is systematically analyzed from both experimental and theoretical aspects,and its application prospect in the field of infrared nonlinear optics is evaluated.The innovative research work of this thesis includes the following:1.Pb2+cations containing stereochemically active lone-pair electrons and d10 transition metal Ag+cations that easily form different coordination geometries were introduced into metal thiophosphates.The first compound Ag6Pb3P4S16 with a NCS structure was found in the Ag-M-P-S(M=Sn,Pb)system.Ag6Pb3P4S16 is a complex three-dimensional framework formed by[Ag S3]planar triangles,[Ag S4]tetrahedra,[Pb S4]quadrangular pyramids,and[PS4]tetrahedra connected by corner-sharing and edge-sharing.Theoretical calculations show that the optical band gap of Ag6Pb3P4S16 mainly depends on the Ag 4d,Pb 6p,P 3p and S 3p orbitals,and the experimentally measured optical band gap is 2.02 e V.Experimental tests of powder samples show that the second harmonic generation response is 1.2 times that of commercial Ag Ga S2 crystals and can achieve I-type phase matching when the particle size is 200–250?m.The introduction of d10transition metals and cations containing lone-pair electrons into metal thiophosphates provides new insight for functional materials with NCS structures.2.Based on the Rb4Hg2Ge2S8 compound with centrosymmetric(CS)structure,a new strategy to construct a NCS structure of mixed alkali-metals was proposed,and a novel NCS mixed alkali-metal chalcogenide(Na3Rb)Hg2Ge2S8 was successfully synthesized.(Na3Rb)Hg2Ge2S8 crystallizes in polar space group of Pca21,and adapts to a filled cristobalite-type structure with a{[Hg Ge S4]2-}?framework formed by the alternating connection of[Hg S4]and[Ge S4]tetrahedra via corner-sharing and with the mixed-alkali cations in the cavities of the framework.Structural analysis and theoretical calculations indicate that the ordered alternating arrangement of nonlinear reactive groups[Hg S4]and[Ge S4]tetrahedra induced by partial substitution of alkali metals plays an important role in enhancing the second harmonic generation effect.(Na3Rb)Hg2Ge2S8exhibits a strong second harmonic response of 1.4×Ag Ga S2@2090 nm and a wide bandgap of 2.76 e V.This strategy of mixed alkali-metals may shed light on structural modification from CS to NCS for new NLO materials.3.The mixed-anion strategy is performed to synthesize two new metal chalcohalide crystals Cs2In PS4X2(X=Cl,Br)with mixed-anion structure by the reaction of chalcogenide In PS4 and halide Cs X.Both Cs2In PS4Cl2 and Cs2In PS4Br2 belong to the monoclinic P21/n space group,with the[In2P2S8X4]4-zero-dimensional structure formed by the[PS4]3-tetrahedron and the[In S4X2]7-octahedron through edge-sharing.Both crystals are indirect bandgap semiconductors and have large optical band gaps of 2.85 e V and 2.78 e V,respectively.Detailed theoretical calculations show that the[In S4X2]7-mixed-anion group plays a dominant role in its optical properties.The synthesis of mixed-anion crystals and the study of the structure-property relationship in this work provide new insight for the exploration of new functional crystals with large band gaps. |
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