Synthesis And Characterization Of Selenite Nonlinear Optical Crystals Containing Main-Group Octahedra

Nonlinear optical（NLO）crystals can expand the wavelength output range of solid-state lasers through the second-harmonic generation（SHG）response,and have important applications in many military and civil fields.However,NLO crystals can exhibit NLO properties only if they have a non-centrosymmetric（NCS）structure.Metal selenites,i.e.,oxide materials containing a Se⁴⁺cation,have become one of the ideal material systems for exploring NLO crystals because Se⁴⁺cation has the stereoactively active lone-pair（SCALP）and is more likely to form a local asymmetric coordination environment with strong distortion,which can be more conducive to the formation of NCS structures.Compared with present studies introducing some cations susceptible of Second Order Jahn-Teller（SOJT）effect,including the d⁰ transition metal（TM）cations and other cations with SCALP(Pb²⁺,Bi³⁺,etc.),main-group metal cations have greater electropositivity and no d-d electronic transition,especially the main group Ga³⁺,In³⁺,Sn⁴⁺and other cations have octahedral coordination configurations similar to the d⁰ TM cations,which can increase the material band gap and laser damage thresholds.In this paper,the author designed and synthesized selenite NLO crystals by introducing the main group octahedra using hydrothermal method.After some experimentation,a series of novel compounds including Ga₂（Se₂O₅）₂（HSe O₃）（H₂Se O₃）F and AGa₃（Se O₃）₂（Se₂O₅）₃（A=NH₄,Rb,Cs）have been obtained,and their properties have been characterized.The research work is summarized as follows:1.F^-anion regulation realizes the transformation from centrosymmetric（CS）to NCS structure,and two gallium selenite crystals were synthesizedOn the basis of the CS compound Ga（Se₂O₅）（HSe O₃）（compound 1）,the first NCS gallium selenite fluorides containing three kinds of Se-O groups,namely,Ga₂（Se₂O₅）₂（HSe O₃）（H₂Se O₃）F（compound 2）has been successfully synthesized by introducing bifunctional F^-anions.Compound 2 crystallizes in the polar space group Pna2₁ and possesses an unprecedentedly lantern-like polar construction units[Ga₂F（Se₂O₅）₄（HSe O₃）₂]^5-.These units adopt well-ordered alignments along the c axis promoting good optical properties including a moderate SHG response（1.4×KDP）,suitable birefringence（0.037 at 1064 nm）and wide band gap（4.71 e V）.Besides,the synthesis of compound 2 also shows the importance of introducing bifunctional F^-anions for the synthesis and design of polar and NCS compounds,which will provide an effective strategy for designing other novel NCS polar materials.2.Synthesis and characterization of alkali metal gallium selenite compoundsAlkali metal cations have rich and flexible coordination modes due to their inherent ionic properties and ionic radii,which can regulate the crystal structures.We combine alkali metals with gallium oxygen octahedra selenite,and synthesize three NCS and two CS alkali metal gallium selenite AGa₃（Se O₃）₂（Se₂O₅）₃（A=NH₄,Rb,Cs）（compounds 3-5）,AGa₂（Se O₃）₂（HSe O₃）₃（A=Na,Rb）（compounds 6-7）,respectively.Compounds 3-5 crystallize in NCS space group P62c.Compound 5 is the first alkali metal gallium selenite that can realize phase matching,which exhibits the SHG respond（1.2?KDP）and a wide band gap（4.04 e V）.Compounds 6 and 7 are stoichiometrically equivalent materials,they share a similar bonding network that is composed of distorted Ga O₆ octahedra and asymmetric Se-O groups.However,they crystallize in different space groups,the structural analysis suggests that the size of alkali metal cations affects the macroscopic centricities.3.Synthesis and characterization of alkali metal tin selenite compoundsBy introducing the main group octahedra Sn O₆ into alkali metal selenite,we obtained successfully two alkali metal tin selenites compounds:Li₄Sn（Se O₃）₄·2H₂O（compound 8）and Na₅Sn₂（Se O₃）₆（HSe O₃）（H₂Se O₃）（compound 9）.Compound 8crystallizes in the NCS space group C2,which is the first NCS alkali metal tin selenite and has a large birefringence（0.177@2090 nm）.Compound 9 contians three Se-O groups,which has a unique 2D[Sn₂（Se O₃）₆]_∞layer in its structure.The synthesis of these two compounds opened up the first study of the alkali metal tin selenite.