High-Throughput Screening Novel Mid-Infrared Nonlinear Optical And Birefringent Crystals On First-Principles

Nonlinear optical and birefringent crystals exert a crucial role in laser technology,which is extensively used in fields of frequency conversion,information storage,fiber-optic communications,light modulators,military,etc.In the mid-infrared region,current commercially available nonlinear optical materials are AgGaQ₂（Q=S,Se）and ZnGeP₂.However,their application ranges are seriously limited by inherent defects including narrow band gaps,low laser damage thresholds,low thermal conductivity for AgGaQ₂,and harmful two-photon absorption at 1 μm for ZnGeP₂.Meanwhile,the exploration of new candidate materials remains urgent.In addition,few materials can extend the birefringence beyond 0.3 with visible transmittance.Therefore,exploring novel mid-infrared nonlinear optical crystals with excellent performances-especially balance wide band gap and large nonlinear optical coefficients,and large birefringent materials（birefringence Δn>0.3）,has become one of the important points in the laser technology field.Herein,we studied the structure-property relationships of the four kinds of mid-infrared nonlinear optical material systems and large birefringent layered non-centrosymmetric chalcohalides through first-principles calculations.The main work and results include the following five sections:（1）Heavy metal oxides can be avoided the high absorption in the mid-infrared region due to intrinsic vibrations,and become an emergent candidate for midinfrared nonlinear optical materials.However,the ternary d⁰-transition metal oxides as mid-infrared nonlinear optical materials have not been systematically investigated by the first-principles.Herein,taking ternary d⁰-transition metal oxides as a research subject,we collected 94 non-centrosymmetric compounds in the inorganic crystal structure database for screening excellent mid-infrared nonlinear optical crystals.Among them,seven compounds are highlighted:HT-La₂Ti₂O₇,ZrMo₂O₈,Rb₃V₅O₁₄,β-Tl₃VO₄,La₂MoO₆,WO₂Cl₂,and WOCl₄.They have a good balance between the wide band gaps 3.01-4.33 eV and large nonlinear optical coefficients of 0.5～0.7 × AgGaS₂(d₃₆=13.4 pm/V),simultaneously possess moderate birefringence of 0.055～0.438 at 1064 nm.（2）Introducing the halogen elements with large electronegative into the nonlinear optical materials can broaden the band gap.We have respectively studied the d⁰-transition metal fluorooxo-functional groups and chalcohalides as nonlinear optical active groups and emergent systems for the design of mid-infrared nonlinear optical materials.It is indicated that the microscopic properties of the d⁰-transition metal fluorooxo-functional groups are comparable to those of the classical tetrahedra MS₄（M=Al,Ga,Si,Ge,P）.The first-principles calculations demonstrate that the alkali/alkaline earth metals d⁰-transition metal oxyfluorides exhibit wide band gaps（all the band gaps>3.0 eV）and large birefringence（Δn>0.07）.The alkali metals d⁰-transition metal oxyfluorides have been partially verified by experiments.We also have systematically analyzed the structureproperty of non-centrosymmetric chalcohalides.The results show that chalcohalides both containing alkali metals and transition metals could be potential applications in mid-infrared nonlinear optical materials,which exhibit excellent nonlinear optical performances.（3）Generally,heteroleptic tetrahedra MO_xT_4-x exhibit larger microscopic properties than those of normal oxy-tetrahedra MO₄（M=B,P,S,Si）,which will improve the nonlinear optical coefficients or birefringence in a crystal.The heteroleptic tetrahedra BOxF_4-x（x=1,2,3）,PO_xF_4-x（x=2,3）,SO₃F,SiO_4-xN_x（x=1,2,3）,and PO_4-xS_x（x=1,2,3）can be utilized to design and explore nonlinear optical or birefringent materials.In addition to the introduction of the F or N atoms into MO₄ tetrahedra,the S or OH/NH₂ monomers also can be introduced into MO₄ to generate MO_xT_4-x tetrahedra.According to the structure-property relationships of known compounds with MO_xT_4-x tetrahedra,enriching and expanding MO_xT_4-x tetrahedra that can be used to explore photoelectric functional materials,including POS（NH₂）₂,PO₂S（NH₂）,AsO_4-xS_x（x=1,2）,PO_4-xN_x（x=1,2,3）,GaO_4-xS_x（x=1,2,3）,GeQ_4-xT_x（Q=O,S;T=OH,N,NH₂;x=1,2,3）,SO₃T（T=F,NH₂,S）,SiOS₃,ZnO_4-xS_x（x=2,3）,and BO₃N.Among them,the heteroleptic tetrahedra AsO_4-xS_x（x=1,2）,GaO_4-xS_x（x=1,2,3）,GeO_4-xS_x（x=1,2,3）,and ZnO_4-xS_x（x=2,3）are potential mid-infrared nonlinear optical active genes.（4）Nitrides,in which P in pnictides is substituted by N.Compared with chalcogenides and pnictides,nitrides might exhibit wide band gaps and large nonlinear optical coefficients,simultaneously owning high thermal properties.Firstly,we screened out a promising mid-infrared nonlinear optical material in 34 asymmetric nitrides by first-principles calculations,namely,nitride MoZn₃N₄ with a LiGaS₂-like structure.Simultaneously,the high-throughput prediction strategy has been implemented,and a series of predicted nitrides in the A^Ⅱ-M-N（A^Ⅱ=Mg,Ca,Sr,Ba,Zn;M=Si,Ge）systems with the stoichiometric ratios 1:6:8,1:7:10,2:5:8,5:2:6,and 1:1:2 are discovered.Among them,six diamond-like structure nitrides with a stoichiometric ratio of 1:1:2 are highlighted,i.e.,Pna2l-A^ⅡSiN₂（A^Ⅱ=Ca,Sr）,Pna2₁-A^ⅡGeN₂（A^Ⅱ=Sr,Ba）,and I42d-A^ⅡGeN₂（A^Ⅱ=Mg,Ca）.The calculated results indicates that the nitrides MoZn₃N₄,Pna2l-A^ⅡSiN₂（A^Ⅱ=Ca,Sr）,Pna2₁-A^ⅡGeN₂（A^Ⅱ=Sr,Ba）,and I42d-A^ⅡGeN₂（A^Ⅱ=Mg,Ca）as mid-infrared nonlinear optical candidates can realize targeted balance properties between the wide band gaps and large nonlinear optical coefficients,as well as moderate birefringence,high Debye temperatures favor high thermal conductivity.（5）Based on the first-principles high-throughput calculation,we developed a birefringence screening and analysis system to explore optical materials with large optical anisotropy.The layered non-centrosymmetric chalcohalides Hg₃AsQ₄X（Q=S,Se;X=Cl,Br,I）with a strong birefringence（Δn=0.391～0.493 at 1064 nm）were screened through the high-throughput screening method.Their birefringence exceeds that of the archetypal birefringent materials such as CaCO₃,TiO₂,YVO₄,and α-BaB₂O₄.Based on the analysis of the electron localization function and the Born effective charges,it is concluded that the large birefringence originates from the synergism of the novel birefringent materials motifs,i.e.,the d¹⁰ transition metal cation Hg²⁺,the layer of lone pair electrons of the As³⁺cation,and the mixed anions Q^2-/X^-（Q=S,Se;X=Cl,Br,I）.This dissertation will provide insight into exploring and designing novel midinfrared nonlinear optical materials and large birefringent materials.