| Chalcogenides have diverse structure and performance diversity.In the field of infrared photoelectric functional crystal materials,a series of nonlinear optical crystals with excellent performance such as AgGaS2,AgGaSe2,BaGa4S7 and BaGa4Se7 have appeared in chalcogenides.Nevertheless,the above-mentioned crystals cannot fully satisfy the current development of laser technology,and it is still necessary to continue to explore and develop new infrared photoelectric functional materials with excellent performance.In this paper,in order to enrich the structural diversity of chalcogenides and control the optical properties of materials,we are trying to simultaneously introduce alkali metals and alkaline earth metals without dd and ff electronic transitions into chalcogenides,which are developed in the A2BCD4 and ABCD3 families Four new chalcogen compounds,Li2MgGeSe4,Li2MgSnSe4,Li2MgSnS4 and LiMgGeSe3,have been systematically studied on their structure and properties.The main research contents are as follows:(1)Alkali metal/alkaline earth metal chalcogenides of diamond-like structure Li2MgSnSe4,Li2MgGeSe4 and Li2MgSnS4.Select the diamond-like structure that is conducive to the uniform arrangement of the groups as the template,use the four-coordinated alkali metal Li and alkaline earth metal Mg to replace the monovalent and divalent metal cations in the A2-B-C-Q4 system diamond-like structure,and adopt a closed system The solid-phase reaction method was used to obtain three examples of composite alkali metal alkaline earth metal quaternary chalcogenides with novel structures:Li2MgGeSe4,Li2MgSnSe4 and Li2MgSnS4.The three examples of compounds are isostructural compounds,all crystallized in the non-centrosymmetric space group Pmn21(No.31 space group)in the orthorhombic system.The unit cell parameters are:Li2MgSnSe4,a=8.402(14)(?),b=7.181(12)(?),c=6.728(11)(?),Z=2;Li2MgGeSe4,a=8.2961(7)(?),b=7.0069(5)(?),c=6.6116(6)(?),Z=2;Li2MgSnS4,a=7.998(13)(?),b=6.826(11)(?),c=6.378(10)(?),Z=2.In these compounds,each metal cation atom is connected with 4 S/Se atoms to form a tetrahedral element,and these elements form a diamond-like structure by sharing the apex S/Se atoms with each other.Based on statistical analysis,the above three compounds are the first found in the A2-B-C-Q4 family of alkali metal alkaline earth metal composite diamond-like structure compounds,which enrich the structural diversity of the A2-B-C-Q4 family chalcogenides.However,the compound of the system absorbs more moisture,so the optical performance test of the experiment has certain challenges.The theoretical calculation results show that Li2MgSnSe4 is a direct band gap compound with a theoretical band gap of 2.33 eV.At the same time,studies have shown that by introducing halogen elements into chalcogen compounds,the optical properties of the materials can be further adjusted,such as increasing the band gap of the materials.(2)Synthesis,structure and performance characterization of a new alkali metal/alkaline earth metal composite quaternary selenide LiMgGeSe3 with large birefringence.Using a closed system solid-phase reaction method,we synthesized the first compound of the A-B-C-D3 family,LiMgGeSe3.The compound crystallizes in the centrosymmetric space group P(?)1m(No.162 space group),and the unit cell parameters are a=6.5809(9)(?),b=6.5809(9)(?),c=6.9216(14)(?),Z=2.In the crystal structure of LiMgGeSe3,the two-by-two bonding structure of Ge atoms are connected to each other by sharing Se atoms.Mg atoms and Li atoms are embedded in the voids of the framework structure to achieve charge balance.It is worth mentioning that the compound presents a nested six-membered ring structure on the c direction.The results of experiments and calculations show that LiMgGeSe3 achieves a balance between band gap and large birefringence.Moreover,the band gap of the compound is mainly determined by the Ge-Se covalent bond,and the introduction of Mg has a positive effect on the band gap of the compound.Due to the layered characteristics of the compound,the structure exhibits strong anisotropy,and the theoretical birefringence of LiMgGeSe3 is 0.372@1064nm.Such a large birefringence is rare in chalcogen compounds which do not contain transition metals.Theoretical analysis show that the birefringence of the compound is mainly derived from the unique Ge-Ge bond and[GeSe3]element,[LiSe6]element and[MgSe6]element have almost no contribution.In addition,LiMgGeSe3 can exist stably in the air,does not absorb moisture,and is expected to be used in the next generation of infrared birefringent crystal materials. |
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