Synthesis,Structure And Properties Of Deep Ultraviolet Beryllium Phosphate Optical Crystal

The deep ultraviolet optical crystal material plays an important role in all-solid-state laser.They expand the original wavelengths range and break the limitations of the development of technology,making laser technology better applicable to various fields.However,there are few deep-ultraviolet crystals which can be used in commercial applications,so the high-temperature solid-phase synthesis method is adopted and we introduce the BeO₄tetrahedra which have good performance to penetrate the deep ultraviolet and the alkali metal cations which are in the absence of d-d and f-f transitions into the phosphate system.The structure and performance of them are studied,and the conclusions are as follows:?1?Two kinds of new deep ultraviolet optical crystals,LiCsBeP₂O₇ and K₂BeP₂O₇,were designed and synthesized.LiCsBeP₂O₇ crystallizes into the orthorhombic Pbcm?No.57?space group.It shows a three-dimensional structure formed by the two-dimensional anion layer sharing coners which consists of the four membered ring[Li₂P₂O₁₂]^12-and the[Be₂P₂O₁₂]^10-ring.The cation Cs⁺is filled between the layers to balance the charge.Interestingly,in the process of four-membered ring connections,two different sizes of Be-O-Li rings are formed.K₂BeP₂O₇ crystallizes into the monoclinic P2₁/c?No.14?space group.This compound exhibits a one-dimensional chain formed by the three-membered ring[BeP₂O₉]^6-sharing comman edge.The cation K⁺is filled between the chain to balance the charge.The two compounds both have good thermal stability,and the ultraviolet cut-off edge are less than 200nm which can be used as the potential deep ultraviolet optical crystal materials.?2?Four kinds of deep ultraviolet nonlinear optical crystal materials LiM₄Be₄P₇O₂₄4 and LiMP₂O₆?M=Cs,Rb?were designed and synthesized.Their structures and properties were described and compared.The results are as follows:LiCs₄Be₄P₇O₂₄4 and LiRb₄Be₄P₇O₂₄4 are all crystallized into the polar tetragonal P222₁?No.17?space group.They all show a three-dimensional skeleton formed by a two-dimensional anion layer with a 12-membered ring mutually linking by sharing corners which consists of two new monomers[LiBeP₄O₁₇]^11-and[Be₃P₃O₁₇]^13-.The cation Cs⁺is filled in the voids of the anion skeleton to balance the charge.LiCsP₂O₆,LiRbP₂O₆ was synthesized by Pan's team in 2018,but it is different from our method.They are crystallized into a non-central symmetry tetragonal Fdd2?No.37?space group.They can be described as a three-dimensional skeleton formed by a two-dimensional anion layer with a 12-membered ring mutually linking by sharing corners which consists of two new monomers[Li₂P₄O₁₇]^12-with different connections.The cation Cs⁺is filled in the voids of the anion skeleton to balance charge.Interestingly,the space group of the four compounds changed after tetrahedral substitution,there may be because the compound energy after the tetrahedral substitution becomes lower,resulting in a change of the space group.It is worth noting that from the difference in bond length and bond angle,we analyze the dipole moment change of the compound after substitution.Powder frequency doubling signal test of LiM₄Be₄P₇O₂₄4 and LiMP₂O₆?M=Cs,Rb?show that the second harmonic generation?SHG?response are about 0.3times KDP and 0.4 times KDP in the visible region.The thermal properties indicates that four compounds have better thermal stability.Solid UV-visible diffuse reflectance spectroscopy indicates that their UV cutoff edges are below 200 nm,indicating that four compounds have potential as deep UV nonlinear optical crystals.The dipole moment calculation results show that the weak SHG of the four compounds are mainly contributed by the twisted CsO_n?RbO_n?polyhedron and the PO₄ asymmetric polyhedron.Theoretical calculations further verify that the main contribution of SHG are CsO_n?RbO_n?polyhedron and PO₄ tetrahedron.