| Nonlinear optical crystal materials have attracted wide attention because of their unique properties.After frequency doubling,frequency combination,frequency difference and other processes,they can change the laser frequency.They are used in many fields,such as laser surgery,laser detection,laser weapons,etc.Nowadays,the synthesis of nonlinear optical crystals with excellent performance and promising commercialization is a hot topic.Among them,frequency doubling crystal materials are the most widely studied class of nonlinear optical crystals.Frequency doubling crystal materials are mainly concentrated in the system of iodate,niobate,phosphate,borate,sulfur compounds and phosphide.Phosphates is a rich source of ultraviolet frequency doubling crystals.In order to expand the types and sources of frequency doubling crystal materials,researchers began to pay attention to research systems similar to phosphate structures.For example,HPO32-produced by phosphite ionization has si1ilar tetrahedral structural units and transmission range as PO43-,but it has greater polarization anisotropy.Therefore,phosphites system is also a potential treasure house of frequency doubling crystal materials.At present,only SnHPO3 and Pb2(HPO3)(NO3)2 crystal materials with frequency doubling effect of inorganic phosphites are available.Phosphites is a common research system of porous materials.This topic is based on the M06 octahedral unit,which is easy to form porous structure to synthesize inorganic metal phosphites frequency doubling crystal materials.It is combined with theoretical calculation to carry out in-depth research on its frequency doubling effect.The main research contents and results are as follows:(1)Two kinds of indium phosphites,In2(HPO3)3(H2O)(1)and In2(HPO3)3(H2O)3·(H2O)(2),were obtained by simple hydrothermal synthesis,both of which were crystallized in P21 polar space groups.In the In2(HPO3)3(H2O)structure,In(Ⅰ)O6 and In(2)O6 octahedrons are cohemally connected to form an isolated In2O10 dimer,which is interlinked with HPO32-to form the entire three-dimensional structure.In I In2(HPO3)3(H2O)3·(H2O),InO6 octahedron and HPO3 tetrahedron share vertices to form a neutral {In2(HPO3)3(H2O)3}∞ three-dimensional framework.These two compounds have moderately sized birefringence of 0.027 and 0.021 at 1064 nm,respectively,enabling phase matching.Under the laser irradiation of 1.064 μm,the frequency doubling effects of their maximum sizes are 0.17 times and 0.2 times of KH2PO4(KDP),respectively.(2)Compound 1 and Compound 2 are three-dimensional frame structures.On the basis of the first two,a new phosphite NLO crystal material NH4[In(HPO3)2]was synthesized by hydrothermal method using the strategy of mixed cations.The compound was crystallized in a non-centrosymmetric P63mc space group with two-dimensional layered structure.In the structure of the compound,two InO6 octahedrons and two HPO32-are interlinked to form{[In(HPO3)2]-}∞ anion layer,and NH4+acts as a cation between the skeleton to balance charge.NH4[In(HPO3)2]showed a moderate frequency doubling effect(0.3 × KDP)and high birefringence(0.098@1064 nm),which increased the frequency doubling effect and birefringence of the compound.In addition,NH4[In(HPO3)2]can be matched and has a suitable through-window.(3)Since the frequency doubling effect of the first three compounds was not strong enough,we started with the MO6 octahedron unit and replaced the InO6 octahedron with the octahedron unit formed by rare earth cations with more significant distortion.Compound BaSc2(HPO3)4·(H2O)2 was formed by simple hydrothermal synthesis.The crystallization of the compound is associated with the non-centrosymmetric space group Pn of the monoclinic system,and they are connected to the ScO6 octahedron by way of angular sharing,forming two different octa-membered {[Sc(HPO3)4]5-}∞ layers.The {[Sc(HPO3)4]5-}∞ layers are connected to each other by shared vertices to form the entire three-dimensional anion{[Sc2(HPO3)4]2-}∞ framework.Ba2+and water molecules are present in the pores to balance the charge and stabilize the structure.The frequency doubling effect of BaSc2(HPO3)4·(H2O)2 is close to KDP and has moderate birefringence(0.036@1064 nm)to achieve phase matching.By theoretical calculation,the twisted ScO6 octahedral element makes a great contribution to the enhancement of SHG effect.This work provides a new idea for exploring phosphites nonlinear optical crystal materials. |
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