Investigation On Synthesis And Performances Of A₂BaM^?Q₄(A=Cu,Ag;M^?=Si,Ge,Sn;Q=S,Se)Chalcogenides

Infrared nonlinear optical?IR NLO?materials in laser have drawn more and more attention because of their wide applications in civil and military regions including laser guidance,long distance laser communication,laser radar and cosmetic laser surgery etc.Commercially applied IR NLO materials still exhabite a series of performance defects,such as low laser damage threshold and two-photon absorption,which greatly limits their application.Therefore,exploring new IR NLO materials with excellent properties have drawn more and more attention in the world.Through the investigation,it is in favour of obtaining large band gap,wide IR transmission region and diversity of noncentral symmetrical structure type through introducing alkaline or alkaline-earth metal in experimental system.Actually,only a few metal sulfides can satisfy both of the above conditions,and most of them contain alkali metal or alkaline earth metal elements.In addition,their structures usually exist[M^IVQ₄]tetrahedra(M^IV=Ga,In,Si,Ge and Sn;Q=S and Se).Therefore,exploring alkaline earth sulfides and performance study means really much for designing and synthesizing new IR NLO materials.Based on the above consideration,to aim at obtaining infrared nonlinear optical materials with wide band gap and strong second-harmonic generation?SHG?,the[M^IVQ₄]tetrahedron and alkaline earth metal Ba element were successfully introduced into the research system.As a result,Cu₂BaM^IVQ₄(M^IV=Si,Ge and Sn;Q=S and Se)and Ag₂BaM^IVSe₄(M^IV=Si,Ge and Sn)compounds were successfully synthesized,and their structures and properties were also analyzed and characterized.More importantly,the effect of monovalent A cations on the structural transformation of the A₂BaM^IVQ₄(A=Li,Na,Cu,and Ag;M^IV=Si,Ge,and Sn;Q=S and Se)system was analyzed for the first time.?1?Cu₂BaM^IVQ₄(M^IV=Si,Ge,and Sn;Q=S,Se):synthesis,crystal structures,optical performances and theoretical calculationsFive non-centrosymmetric?NCS?quaternary metal chalcogenides,Cu₂BaSiSe₄,Cu₂BaGeS₄,Cu₂BaGeSe₄,Cu₂BaSnS₄,and Cu₂BaSnSe₄,were successfully synthesized by solid-state reaction.They crystallized in the three different space groups:Ama2 for Cu₂BaSnSe₄,P3₁2₁ for Cu₂BaGeS₄ and Cu₂BaGeSe₄,and P3₂2₁for Cu₂BaSiSe₄ and Cu₂BaSnS₄.Note that Cu₂BaGeS₄ and Cu₂BaGeSe₄ show the mirror symmetrical structures to those of Cu₂BaSiSe₄ and Cu₂BaSnS₄.In comparison with their structures,it can be found that the [Cu Se₄] units are connected together to form a two-dimensional?2D?layer structure in Cu2 Ba Sn Se4,which is different from the 3D framework structure formed by the interlinked[CuQ₄]?Q=S,Se?units in other four title compounds.In addition,Cu₂BaSnSe₄ exhibits only one type of tunnel structure with the isolated[BaSe₈]units existing in each tunnel,which is also different from the other title compounds?two types of tunnels with the isolated[BaSe₈]units and[BaSe₆]_n chains located?.The interesting structural changes also indicate that slight change of cation size would result in different structure features,and future structure prediction should devote considerable attention to the different chalcogen atoms.Moreover,important optical properties?optical bandgap,infrared?IR?absorption edge,second harmonic generation?SHG?response?of the title compounds were systematically investigated.Among them,IR and Raman spectra indicate that all of them exhibit the wide IR absorption edges?²2?m?.Powder SHG measurement shows that Cu₂BaSnS₄ possesses a good SHG response about 1.6times that of benchmark AgGaS₂?AGS?at the particle size 55⁸8?m.All results indicate that Cu₂BaSnS₄ can be expected as a potential IR nonlinear optical?NLO?candidate.Theoretical calculation was also used to analyze the structure–property relationship and their electronic structures and origin of NLO effect were studied in detail.?2?Ag₂BaM^IVSe₄(M^IV=Si,Ge,and Sn):the synthesis and performance characterization of three four-element sulphides containing Ag.Ag₂BaSiSe₄,Ag₂BaGeSe₄ and Ag₂BaSnSe₄ were synthesized by high temperature solid-state reaction,and their structures and related optical properties were analyzed and tested systematically.See from their structures,they are all composed of 2D[AgSe₄]layer,[M^IVSe₄]tetrahedron and[BaSe₈]polyhedron.Optical performance tests indicated that these three compounds have infrared transmission range of2.5²5?m.The diffuse-reflectance UV-vis-NIR spectra of Ag₂BaM^IVSe₄ were measured,and the spectral results show that the experimental band gaps are 1.83 eV for Ag₂BaSiSe₄,1.57 eV for Ag₂BaGeSe₄,and 1.42 eV for Ag₂BaSnSe₄,respectively.The calculations of SHG density indicate that their origins of SHG effects are mainly derived from the cooperative contribution of the[AgSe₄]and[M^IVSe₄]tetrahedra.?3?Comparison and performance analysis of compounds in A₂BaM^IVQ₄(A=Li,Na,Cu and Ag;M^IV=Si,Ge and Sn;Q=S and Se).A series of new IR NLO materials have been discovered in the A-Ba-M^IV-Q system(A=Li,Na,Cu,and Ag;M^IV=Si,Ge,and Sn;Q=S and Se),and they undergo interesting structural transformation with different element substitution except Li analogues.The detailed analysis on the structural changes and properties comparison was investigated in the A-Ba-M^IV-Q system and the results indicate that the distortion degrees of different[AQ₄]tetrahedra play a critical role to cause the structural transformation with the M^IV or Q elements substitution.More importantly,it can be found that the structural changes have the close relationship with the distance d_?A-A?between adjacent A cations in the A₂BaSnSe₄ system,which makes the four-membered rings formed by edge-sharing[BaSe₈]units change from the square to rhombus with the increase of d_?A-A?.The properties comparisons?band gap and NLO effect?in this system have been also systematically studied.The results indicate that the optical band gaps show the decreasing tendency with different A atoms along the Na,Li,Cu,to Ag analogues,and the SHG effects of selenides are larger than those of sulfides.The above results show that the introduction of[M^IVSe₄]tetrahedra in the research system is beneficial to obtain compounds with non-central symmetry structures,and the monovalent A cations in the A₂BaM^IVQ₄ system can regulate the structures and properties.It can provide strategies for synthesizing new functional materials.