Synthses,structures And Optical Properties Of Several New Transition Metal Iodates

Nonlinear optical（NLO）crystals are the core materials for solid-state lasers to achieve wavelength expansion and have important applications in many fields related to national economy and people’s livelihood,however,a prerequisite for NLO crystals is that the material crystallizes in a non-centrosymmetric space group.Metal iodates contain stereochemically active lone-pairs electrons that can produce greater distortion of structures,which makes it easier to obtain non-centrosymmetric crystal structures.In particular,the introduction of d⁰ transition metal cations(Mo⁶⁺,V⁵⁺,Nb⁵⁺,Ti⁴⁺,etc.),which are prone to second-order Jahn-Teller distortion,into the iodate structures can further increase the iodate crystal structural distortions,thereby enhancing the second harmonic effect of the materials.According to calculations by Halasyamani et al.at the University of Houston,USA,the three ions Mo⁶⁺,V⁵⁺,and Nb⁵⁺possess greater structural distortion among the d⁰ transition metal cations and are ideal choices for the design of non-centrosymmetric iodate structures.Therefore,in this thesis,the author takes the iodates containing Mo⁶⁺,V⁵⁺and Nb⁵⁺cations as the research object,1)achieved the structural design of transition metal iodates with different crystal structures in Mo-containing iodates by modulating the orientation of basic structural units,and synthesizedγ-KMoO₃（IO₃）with the largest second harmonic response in this series of compounds.2)To further increase the distortion of transition metal iodates,F^-ions with the largest electronegativity were introduced into Mo⁶⁺,V⁵⁺,and Nb⁵⁺iodates to modulate the structural composition,and six new crystals of F-containing transition metal iodates were synthesized,and the influence laws of F^-ions on the distortion of different d⁰ transition metal coordination octahedra were investigated,providing a theoretical and experimental basis for the structural design of new F-containing iodates,the contents of this paper are as follows:1.Synthesis and performance study of a series of iodate containing Mo homogeneous polymorphic phases based on the modulation of structural units orientationAs is well known,the orientation of structural units is crucial for the NLO properties of materials.In this work,the authors achieved the first synthesis of transition metal iodate materials with four homogeneous polymorphic phases by adjusting the arrangement ofΛ-shaped cis-MoO₄（IO₃）₂ units.Among the four crystalline forms of KMoO₃（IO₃）,α-andβ-KMoO₃（IO₃）crystallize in the Pbca and P2₁/n space groups with nonpolar layered structures,respectively,whileγ-andδ-KMoO₃（IO₃）crystallize in the Cc and Pna2₁ space groups,respectively,displaying polar framework structures,and both have good linear and NLO properties,including large second harmonic generation response（12×and 6.6×KDP）,wide band gap（3.40 e V and 3.34 e V）,and wide infrared transparency region（～10.2μm and 10μm）.Structural analysis of the four phases of KMoO₃（IO₃）shows that the orientation of theΛ-shaped cis-MoO₄（IO₃）₂ unit has an important influence on the structures and properties of the material.This work provides a feasible way to design new NLO crystals.2.Study on the effects of F^-ions on the structures and properties of different d⁰transition metal iodatesBy introducing F^-ions into Mo⁶⁺,V⁵⁺and Nb⁵⁺iodates and adjusting their structural compositions,six F-containing transition metal（TM）iodates were synthesized,including four d⁰ TM iodate-fluorides:Ba₃[（Mo₂O₅F₄）（IO₃）₄（H₂O）₂],Ba₃[（Nb₂O₃F₆）（IO₃）₄（H₂O）₂],Ba₂F[VO₂F₂（IO₃）₂]and Ba[VO₂F₂（IO₃）],and two d⁰ TM fluoroiodates:K₂（Nb OF₄）（IO₂F₂）and Ba[VO₂（IO₃F）]₂.The influences of F^-ions on the structures and properties of different d⁰ TM iodates were studied.It was found that the introduction of F^-ions contributes to increase the band gap of the materials and different distortion of TM octahedra,and the reasons for the differences in the number of F^-ions in octahedral coordination of different transition metals were analyzed according to Lewis acid-base theory.These laws are explained in detail in the paper,This provides theoretical guidance for the structural design of novel F-containing TM iodates.