Exploration And Research Of New Birefringent Crystals Based On C₃N₃O₃ Structural Group

The planarπ-conjugated anionic group（C₃N₃O₃）^3-has a shorter in-ring bond length and a more uniform charge distribution,and usually can bring greater optical anisotropy and higher second-order polarization.It is considered that It is an excellent structural unit for birefringent or nonlinear optical materials.And（C₃N₃O₃）^3-groups can be synthesized from linearπ-conjugated anion groups（CNO）-with sp hybrid orbitals through ring trimerization.Therefore,this paper mainly systematically studies these two anionic groups and synthesizes a series of new functional materials.The results are as follows:（1）A new phase of cyanurate Ba₃（C₃N₃O₃）₂ crystal was synthesized by a solid state cyclotrimerization reaction.This new phase has the same chemical formula as Ba₃（C₃N₃O₃）₂,but the space group is different.It cystallizes in space group R3,the c-axis is only half of the first type,and featuring a layered structure stacked from the ∞²[Ba（C₃N₃O₃）₂]^2-double layers along c direction.Ba3（C3N3O3）2 exhibits wide ultraviolet transparency and large optical anisotropy ofΔn=0.32 at 800 nm,two times than that of benchmark birefringent crystal calcite.The discovery of Ba3（C3N3O3）2opens the novel pathway for designing novel ultraviolet birefringent crystals in metal cyanurates.（2）Four novel cyanates BaCNOX（X=Cl,Br,I,and CNO）containing linearp-conjugated anions,were obtained by solid-state metathesis reactions for the first time.Among them,BaCNOCl crystallizes in a monoclinic space group P2₁/m with a=5.5728（6）?,b=4.4917（4）?,c=7.8518（9）?,β=102.319（11）°,Z=2,whereas the remaining three compounds belong to the same orthorhombic space group Pnma with a=8.227-8.713?,b=4.290-4.551?,c=10.986-11.803?,Z=4.The crystal structures of these four compounds exhibiting rich structural diversity ranging from simple to complex when X changes from Cl to Br and I,and then to CNO.In addition,the basic properties of these four compounds were characterized in details.The first principles calculations reveal that all four compounds show giant optical anisotropy of ?n>0.20 at 800 nm owing to the superiority arrangement of linear（CNO）^-p-conjugated groups.（3）For the first time,we grew a new mixed anions compound Ba₂（C₃N₃O₃）（CNO）containing two typesπ-conjugated groups of planar（C₃N₃O₃）^3-six-membered rings and linear（CNO）^-units.This compound exhibits wide bandgap of 4.82 eV and high absorption coefficients among 200-280 nm,indicating it is a potential optical detection material in solar-blind region.