Exploration, Synthesis And Properties Research Of Novel Birefringence/Nonlinear Crystals With Planar Triangular Groups

As a key material in high-tech fields such as microelectronics,optoelectronics,communications,aerospace,and modern military technology,photoelectric functional crystal materials have received worldwide attention.Among them,birefringent crystals can be made into optical polarizers,optical isolators,etc.,while nonlinear crystals can play an important role in broadening the laser output wavelength in solid-state lasers.Currently,many birefringent/nonlinear crystals with excellent performance have been commercialized,but in the ultraviolet/deep ultraviolet region,they are still unable to meet the needs of large-scale applications.According to the anionic group theory,anionic groups play a decisive role in birefringent/nonlinear crystals.π-conjugated planar triangular MO₃（M=B,C,N）groups with high UV transmission,large anisotropic polarizability and microscopic second harmonic generation response are ideal substrates for the design and synthesis of birefringent/nonlinear crystals with excellent performance.In this paper,a series of new compounds have been successfully synthesized around the MO₃ moieties,and their properties have been initially investigated as follows.（1）Two carbonate nitrates Na₃Rb₆（CO₃）₃（NO₃）₂X·6H₂O（X=Br,Cl）were synthesized with[CO₃]and[NO₃]groups as the core:Through a mixed anionic strategy,[CO₃]and[NO₃]groups are rarely combined.Halogen ions（Cl^-,Br^-）and alkali metal ions（Na⁺,Rb⁺）are also introduced to improve UV transmission.The Na⁺ions in the crystal structure of Na₃Rb₆（CO₃）₃（NO₃）₂X·6H₂O rarely have an equatorial oxygen atom structure,which is usually found in divalent metal cations.Benefited from the unique equatorial oxygen atoms of the[Na O₇]polyhedra,the[CO₃]and[NO₃]groups are arranged coplanarly,which facilitates the superposition of each anisotropic polarization rate.The first principle calculations show that their birefringence at 1064 nm is 0.165 and 0.166,respectively,and their UV cut-off edges are228 nm and 230 nm,respectively,as measured by UV diffuse reflectance spectroscopy.These properties suggest that Na₃Rb₆（CO₃）₃（NO₃）₂X·6H₂O are promising UV birefringent crystals.（2）Two new noncentric rare-earth ionic borates K₆NaCaRe₂(B₅O₁₀)₃（Re=Y,Lu）were synthesized with the[BO₃]group as the core:Through the equivalent ion replacement strategy,using K₆Na Ca Sc₂(B₅O₁₀)₃ as a template,the Sc atoms were replaced with Y and Lu,respectively.K₆NaCaRe₂(B₅O₁₀)₃ both crystallized in the R32 space cluster and exhibit a three-dimensional[Re₂(B₅O₁₀)₃]anion structure consisting of[B₅O₁₀]clusters and[Re O₆]octahedra,in which alkali and alkaline earth metal cations occupy cavities and maintain charge balance.The band gaps of K₆NaCaRe₂(B₅O₁₀)₃ are about 5.39 and 5.25 e V calculated from UV diffuse reflectance spectra.Moreover,their structures belong to noncentric space groups and have certain nonlinear effects,which have potential application value.