| Nonlinear optical crystal materials have become a research hotspot in the field of materials science and engineering due to their wide application in microelectronics,communications,and military technology.In particular,in the field of infrared nonlinear optics research,chalcogenides have received widespread attention due to their large nonlinear optical coefficients and wide infrared transmission range.The different metal cations in the compound not only affect the structure of the material,but cause differences in nonlinear optical properties.Therefore,exploring the influence mechanism of the difference of metal cations on the nonlinear optical properties of materials would be beneficial for the design of materials oriented by nonlinear optical performance,and provide a theoretical basis for the exploration and discovery of new infrared nonlinear optical crystal materials with excellent performance.In this work,ternary chalcogenides are used as the research system,based on first-principles methods,and through cation replacement design strategies,the effects of different metal cations(alkali-metal cations and metal cations with d10 electron configuration)on the structural transformation and nonlinear optical property are discussed.The impact of performance changes clarifies the microscopic mechanism of the difference of metal cations on the macroscopic properties of materials,and provides an effective way for the design of infrared nonlinear optical materials with excellent performance.The specific research work is as follows:1.Research on alkali-metal cation replacement and nonlinear optical properties based on the advantage structure template LiInS2The typical infrared nonlinear optical crystal material LiInS2 was selected as the structural design template.By replacing the A-site alkali-metal cations of the same main group,a class of ternary chalcogenides with different A-site alkali-metal cations were successfully obtained—AIn S2(A=Na,K,Rb,Cs).Interestingly,the replaced structure presents two different structure types,of which only NaInS2 maintains the same structure as the parent LiInS2—A?n S2(AI=Li,Na);and A?In S2(A?=K,Rb,Cs)although it maintains the same space group(Pna21)with LiInS2,it exhibits a different structure from LiInS2 due to the difference in the cation coordination number(from 4 to6).The stability of the structure after the replacement was verified by phonon spectroscopy and molecular dynamics simulation,and then the electronic structure and optical properties of the AIn S2 series were calculated,and the effect of the replacement of alkali-metal cations on the structural transformation and nonlinear optical properties of the material were systematically studied.The microscopic influence mechanism of the difference of cations on the nonlinear optical properties of materials was interpreted.The results show that the change of volume is the main reason for the change of the maximum effective second harmonic generation coefficient d32 and the optical band gap.Therefore,the use of volume effect to adjust the two crucial nonlinear optical properties of band gap and second harmonic generation coefficient can be used as an effective method to explore infrared nonlinear optical materials.2.Study on the substitution of metal cations with d10configuration and nonlinear optical properties based on the advantage structure template LiInS2Based on the dominant structural template framework of ILiInS2 and ?AgInS2,the cation replacement was used as a design strategy to replace the metal cation Ag with d10electron configuration and the alkali metal cation Li,and the structural models of IAgInS2 and ?LiInS2 were obtained respectively.We simulated and explored the effects of ILiInS2,IAgInS2,?LiInS2 and ?AgInS2 on performance changes from theoretical calculations,and systematically studied the influence factors of the d10 electron configuration on the optical band gap and the source of nonlinear optical effects.We explained the micro-mechanism of the difference of the A-site cation on the property change.It reveals that an important contribution to the frequency doubling response of cationic Ag with d10 configuration.The results show that the source of the gain of the maximum effective frequency multiplication coefficient d32 not only depends on the d orbital of cation Ag with d10 configuration,but also is closely related to the volume effect of the cation Ag.It is worth noting that compared with the compounds containing the alkali metal cation Li,the second harmonic generation coefficient of the compounds containing the cation Ag with the d10 configuration is more sensitive to the volume change,and d32 changes with the volume more obviously.Therefore,the volume effect of metal cations containing d10 electron configuration has important theoretical significance for the exploration and design of infrared nonlinear optical materials. |
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