Infrared Nonlinear Optical Crystals Based On Chalcogenides Synthesized At Intermediate Temperatures

Infrared（IR）nonlinear optical（NLO）materials can tune the wavelength of high-power lasers to mid-and far-IR ranges,which is able to run across two important atmospheric transparent windows,based on second harmonic generation（SHG）effect such as optical parametric oscillation（OPO）.Thus,the mid-IR lasers have been indispensable in many fields,such as remote sensing,environmental monitoring and spacing security.A good IR NLO material must satisfy the following requirements:Large NLO coefficient,high laser-induced damage thresholds（LIDT）,moderate birefringence for phase matchability,good physical and chemical stability and crystal growth habit.However,commercially available IR-NLO materials are few and still exit some drawbacks,hindering their high-power applications.There is urgent demand the discovery of new mid-IR NLO materials which could satisfy the above optical and other conditions.Generally,high temperature synthesis（temperature（T）>600°C）is used in preparing IR-NLO chalcogenides with simple connection types between NLO-active units,limiting great progress of property.Utilization of metal polychalcogenides flux as reaction media successfully reduce reaction temperature to below 600°C,and obtained four non-centrosymmetric（NCS）quaternary chalcostannate,through the mixing covalent NLO-active motifs with a third metal with appropriate electronegativity.Research of low temperature synthesis involving the utilization of surfactants as solvents to direct the growth of crystalline chalcogenides is unexpected at high temperature synthesis.Synthesized at intermediate temperatures could produce not only NCS chalcogenides that have SHG effect but also four promising candidates for IR NLO applications（Na₂Zn Sn₂Se₆（1）,Na₂Cd Sn₂Se₆（2）,Cs Mn As₃S₆（5）and Rb Mn As₃S₆（6））,which could satisfy the above requirements,particularly high LIDT compared with commercial Ag Ga S₂（AGS）,revealing that the possible of excellent infrared nonlinear optical crystals based on chalcogenides synthesized at intermediate temperatures.The main three achievements are as follows:（1）Utilization of metal polychalcogenides flux as reaction media and reactant successfully obtained two quaternary IR NLO chalcogenides,Na₂Zn Sn₂Se₆（1）and Na₂Cd Sn₂Se₆（2）.Compounds 1 and 2 crystallize in the monoclinic phases with non-centrosymmetric space group of Fdd2.As the main NLO active units,covalent Sn Se₄chains have large contribution to SHG response for compounds 1 and 2,which are about3 and 2.2 times that of Ag Ga S₂.The bonds formed by alkali metal Na and Se primarily show the ionicty,which are benefit to increasing LIDT.Furthermore,incorporating a third d₁₀metals,Zn and Cd,with electronegativity between those of alkaline metal Na and metal Sn,achieve the balance between large NLO efficiency and high LIDT by modulating the covalency and ionicty contributions.Compounds 1 and 2 presents high LIDT of 5.0 and 10.0 times that of the benchmark AGS.In addition,they possess wide IR transparency regions（～0.6-25μm）with no obvious chemical bond absorption and congruent-melt behaviour,suggesting that both compounds are promising candidates for IR NLO applications.（2）Two new selenides,namely,K₂Zn Sn₃Se₈（3）and Na₂Ag₆Sn₅Se₁₄（4）,were obtained by mixing tetrahedron units modulating electronegativities.Two compounds crystallize in the monoclinic phases with chiral space group of P2₁2₁2₁.It is different that compound 3 possesses three-dimensional（3D）structure while compound 4possesses two-dimensional（2D）structure.The synthesis of compound 3 and 4 reveal that mixing NLO-active units motifs is benefit to forming non-centrosymmetric structure,which is more possibly to find new structure to suit IR-NLO crystals.（3）Applying unusual surfactant as solvent,the reaction temperature decreases to low temperature zone,which is too low compared with high-temperature solid-state method.Although the reaction temperature is reduced,the solubility of surfactant and relatively low viscosity facilitate diffusion of reactant,two IR NLO sulfides,Cs Mn As₃S₆（5）and Rb Mn As₃S₆（6）,successfully obtained with enhanced SHG effect by low coordination pyramidal units As S_3.Cycle-polymeric tripyramidal[As₃S₆]^3-of in-phase arrangement are introduced as main NLO-active units for the first time,which are unprecedented in NLO material synthesized by solid-state reaction.Due to these units and highly electropositive metal cation,two compounds 5 and 6 exhibit outstanding optical property,which comprehensive performance are superior to commercially available IR-NLO crystals AGS（strong SHG response as similar as AGS,and high LIDT over ten times that of AGS）.The novel structure in high-temperature solid product also overcome nonphase matchable and low LIDTs of most of narrow gap chalcogenides containing stereochemically active lone pairs（SALP）metal centre pyramids as NLO-active units.Furthermore,the synergistic effect of ionic species and surfactant has been found with deepening of the study about synthesis mechanism.