Research On The Design,Syntheses And Properties Of Novel Silicate/Germanate Optical Functional Crystals

With the rapid development of information technology and the wide application of artificial intelligence,laser technology has been put forward more and more high requirements.Effectively extending laser wavelength range and efficiently modulating laser polarization state has become a hot but difficult problem in scientific research.At present,the practical nonlinear optical（NLO）and birefringent crystal materials mainly focus on borate,phosphate,niobate,titanate,tungstate,molybdate and chalcogenide systems.However,all of them are more or less defective,so it is urgent to develop new optical functional crystal materials.High symmetry and even cubic structure of silicates and germanates in IV-group are easy to induce little second-order susceptibility and weak optical anisotropy,which leads to most existing crystal materials of them with weaker second-order nonlinear optical response and lower birefringence,going against thier application and development seriously.In order to meet this challenge,silicates and germanates are explored to expand the scope of NLO and birefringent crystal and ultimately design and synthesize new optical functional crystals materials with excellent properties in this paper.The specific approach to design and synthesize a series of novel silicates/germanates NLO and birefringent crystal materials is to respectively introduce main-group cation Bi³⁺with lone pair electrons and d⁰ transition metal cation Nb⁵⁺with octahedral coordination,both exhibiting second-order Jahn-Teller effect,as well as large-radius alkali metal ion Cs⁺into the silicates/germanates.（1）The synthesis and properties of silicates with stereochemically active lone pair electron Bi³⁺cationA novel silicate crystal C_s2Bi₂OSi₂O₇ was obtained by introducing main-group cation Bi³⁺with lone pair electrons into the silicates through high-temperature solid-phase reaction method.The crystal crystallized in a non-centrosymmetric space group P6 2c,exhibiting the powder second harmonic generation（SHG）effect of 3.5×KDP and conforming to phase-matching behavior.Moreover,the crystal possessed good thermal stability and melted congruently.Besides,it had a short ultraviolet（UV）cut-off edge(λ_cutoff=279 nm)and the largest birefringence（?n=0.044@1μm）among the NLO lead-free silicates,making it a potential NLO crystal material.（2）The synthesis and properties of germanates with d⁰ electron configuration Nb⁵⁺cationThis work proposed a property improvement strategy with synergetic electron distribution and lattice vibration to introduce d⁰ electronic configuration Nb⁵⁺cation into the low phonon energy germanate and ultimately obtained a new germanate birefringent crystal Cs₃Nb₅Ge O₁₆,which crystallized in a centrosymmetric Pmma space group.The UV-Vis diffuse reflectance spectrum showed an optical bandgap of3.75 e V,implying its high laser damage resistance.Infrared spectrum analysis indicated that the crystal had an excellent mid-infrared transmittance range covering the entire atmospheric window of 3～5μm.In addition,the synergistic effect of the distorted[Nb O₆]octahedra and antiparallel arrangement of[Ge O₄]tetrahedra in Cs₃Nb₅Ge O₁₆ results in a high birefringence of 0.19 at visible wavelength,which is similar to that of the commercial birefringent crystal YVO₄ and is the largest experimental value reported for germanates at that time.The above results demonstate that Cs₃Nb₅Ge O₁₆ is expected to be an outstanding mid-infrared birefringent crystal material.