Infrared Nonlinear Optical Crystal Material Based On BaGa₂Se₄ Was Designed

Second-order nonlinear optical(NLO)crystalline materials can convert lasers into new ones with different frequencies,thus expanding their applications for laser techniques.Effective conversion of laser frequencies needs high-quality NLO crystals,which therefore raises high requirements for the NLO data.The most important data include large NLO coefficient,high laser-induced damage threshold(LIDT),and phase-matchable second-harmonic generation(SHG)behavior.NLO materials in the middle infrared(MIR)region are insufficient to satisfy high-tech laser demands,therefore,to explore MIR NLO crystal materials with excellent performance is the focus of our research.One is to improve the known ones' NLO performances,and the other one is to explore new candidates.Constructing new materials from known ones has become an effective method to explore new NLO crystal materials.Among potential MIR NLO candidates,metal chalcogenides have attracted great attention since many of them possess large NLO effects and wide transparent ranges.Most successful chalcogenides contain MQ4(M=Ga,In,Si,Ge,Sn,Zn,Cd,Hg;Q=S,Se)tetrahedron as NLO functional motif,and also strong positive electrical cations contributing large band gaps.Combining the two chemical components together,we have an interest to design new NLO crystals based on A-MQ4.There is a family of chalcogenides with the formula M?M2Q4(M?=divalent alkali-earth metal,Pb,Sn,and Eu ions;M=Ga,In;Q=S,Se),which have never been studied as NLO materials since all the known ones crystallize in the centrosymmetric(CS)structures.Based on this fact,BaGa2Se4 with CS structure was chosen as the mother compound to explore new IRNLO materials.It is well known that non-centrosymmetric(NCS)structure is the precondition for corresponding SHG behavior,so we tried to break the CS structure of BaGa2Se4 to NCS structures of the derivatives by adopting appropriate substitution strategies at different lattice positions of ions,and then study the new NCS derivatives'structures and NLO properties.The main work addressing this idea is simply summarized below.(1)BaGa2Se4 is crystallized in the CS space group Cccm and has a large optical band gap(3.16 eV).Based on BaGa2Se4,we tried to obtain its NCS structure derivative by making the symmetry center disappeared in the original structure through appropriate substitution strategies.Based on this idea,we attempted to meanwhile introduce S and K partially occupying the lattice positions of Se and Ba,respectively.By changing the Ga-Se bond length in the functional motif GaSe4 tetrahedron,the structure of BaGa2Se4 can be destroyed,and two NCS compounds K0.68Ba0.66Ga2S0.9Se3.1 and K0.63Ba0.73Ga2S1.8Se2.2 were successfully obtained by a high temperature solid state method.Both of them crystallize in the NCS space group I4cm.There is only one type of structural unit existing in the structure of 1,namely,{[Ga(S/Se)2]-}n polyanionic chains along the c direction constructed by the connection of Ga(S/Se)4 tetrahedra via sharing of the S/Se edges.Their NLO performances were evaluated and the results show that under the laser radiation of 2.1 ?m,the SHG effects of 1 and 2 can reach to 0.2 and 0.4 times of that of commercial AgGaS2(AGS),and the phase matchability cannot be achieved.UV Diffuse reflectance and IR spectra show that their band gaps were 3.16 and 3.65 eV,respectively,and they also have a wide infrared transmittance range.Therefore,their NLO properties need to be improved.(2)Based on the result of(1),we tried to optimize the structure and obtain new derivativea with improved NLO performance.Subsequently,alkali metal elements were introduced into the Ba position of BaGa2Se4,and there was no substitution on the Se position.Luckily,two quaternary metal chalcogenides A2-2xBaxGa2Se4(A=Na,K)were successfully synthesized by high-temperature solid-state reactions.Both of them crystallize in the tetragonal NCS space group I4cm.Their NLO performances were systematically studied,and the results show that K0.38Ba0.81Ga2Se4 has a large NLO coefficient and high LIDT with 0.9 and 13.4 times of AGS,respectively;the NLO effect and LIDT of Na0.60Ba0.70Ga2Se4 are 0.3 and 2.4 times of AGS.Specifically,both of them can achieve phase matchability in a certain wavelength range.In addition,no further to optimize the synthetic conditions,their crystals can grow up the millimerer-scale size with good quality.Therefore,those crystals of A2-2xBaxGa2Se4 shows potential to be grown big crystals.(3)The d10 cations(Zn2+,Cd2+etc.)tend to exhibit asymmetric coordination geometries with large distortion for building NCS crystal materials because of the second-order Jahn-eller(SOJT)effect,and it can increase the probability of forming an asymmetric structure,further contributing to obtaining crystalline materials with large NLO coefficients.In this work,two NCS compounds BaZnSnQ4(Q=S;Se)were successfully obtained by a high temperature solid-state method by using the CS BaGa2Se4 as the mother phase,with the d10 cation Zn2+,and Sn element which is easy to form aberrant tetrahedral was introduced to co-substitute.Both of them crystallize in the space group Fdd2,and their structures are featured by a[(ZnSnQ4)2-]n(Q=S;Se)layer composed of ZnQ4 and SnQ4 tetrahedra,and the Ba2+cations are inserted between the layers as a counterweight ion to balance the charge.The experimental result shows that BaZnSnSe4(2)exhibits a larger powder second-harmonic generation(SHG)intensity and a higher laser-induced damage threshold(LIDT),which is about 1.0 and 9.8 times of AGS,respectively;BaZnSnS4(1)has a moderate NLO coefficient and a higher LIDT,which is about 0.6 and 5.7 times of AGS,respectively.More importantly,both of them can achieve type-I phase-matchability behavior.In conclusion,the crystals of BaZnSnQ4 have potential application value in the field of IR NLO field.