Synthesis,Structure Characterization And Electronic Structure Calculation Of Two Alkaline-earth Metal Borates SrB₃O₄（OH）₃·H₂O And Sr₂B₅O₉（OH）·H₂O

Deep ultraviolet（DUV）optical crystals are critical components in optical communications and laser systems.For example,DUV birefringent crystals have important applications in optical isolator,optical polarizer,193 nm photolithography,UV spectrophotometers,DUV Raman spectroscopy and fluorescence spectroscopy;DUV nonlinear optical crystals can be applicated in the semiconductor processing industry,ultra-high resolution photoelectron spectroscopy and laser lithography.Therefore,the DUV optical crystal has become a hot spot in the field of material research.Borate crystals have drawn enormous attention attributable to their wide range of applications as laser hosts,phosphors,electrode materials,birefringent optics and nonlinear optical（NLO）materials.In this paper,the synthesis strategy of alkaline-earth borate was put forward.Two alkaline-earth borates were synthesized by hydrothermal method,successfully.The characterization and theoretical analysis of these two crystals were carried out,and the main contents are as follows:1.SrB₃O₄（OH）₃·H₂O was synthesized by hydrothermal method.In the H3BO3-SrO-H2O system,we have obtained a new alkaline-earth metal borate SrB₃O₄（OH）₃·H₂O.Single-crystal X-ray diffraction analysis reveals that it crystallizes in the centrosymmetric space group P2₁/c,a=6.249（4）?,b=11.459（6）?,c=8.749（5）?,β=108.493（6）°,Z=4.SrB₃O₄（OH）₃·H₂O contains infinite[B₃O₄（OH）₃]^2-chains,B₃O₈ ring connect with each other by sharing O5 atoms to form an infinite one-dimensional polymer borate chiral[B₃O₄（OH）₃]^2-chains along the c axis direction,strontium cations filling the space between these chains.There are inter-chain hydrogen-bonds leading to a compact 3D structure.UV-Vis-NIR spectrum shows that SrB₃O₄（OH）₃·H₂O possesses a wide range of transparency 183-2600 nm and a UV cut-off edge is 183 nm which indicates that it may be applied in the deep ultraviolet region.First-principles electronic structure calculation performed with the density functional theory（DFT）method shows that the calculated bandgaps are 5.76eV,which is in good agreement with the UV-Vis-NIR experimental value 6.15 eV.And it indicates that SrB₃O₄（OH）₃·H₂O is a indirect band gap compound.We also study the linear optical properties of SrB₃O₄（OH）₃·H₂O by calculating refractive indices.It is clear that SrB₃O₄（OH）₃·H₂O is a positive biaxial crystal due to n_z-n_y>n_y-n_x.In the region of 180-2000 nm,the birefringence indices is about 0.060-0.040.In addition,IR spectroscopy,thermal stability and theoretical calculations of SrB₃O₄（OH）₃·H₂O are also reported in this work.2.A noncentrosymmetric borate:Sr₂B₅O₉（OH）·H₂O.A noncentrosymmetric borate,Sr₂B₅O₉（OH）·H₂O,has been synthesized.X-ray diffraction analysis reveals that the compounds were crystallized in the non-centrosymmetric space group C2,a=10.241（5）?,b=8.029（4）?,c=6.399（3）?,V=414.9（3）?³,Z=2,The fundamental building block（FBB）of Sr₂B₅O₉（OH）·H₂O is a B₅O₁₂ double-ring with each ring consisting of a central BO₄ tetrahedron,a non-central BO₄ tetrahedron and a BO₃ triangle sharing corners with each other,B₅O₁₂double-ring further sharing oxygen atoms to form a 3D borate frame structure of（B₅O₉）^3-.The extra-framework Sr²⁺cations and water molecules are located in these channels to compensate the negative charges of the network.The UV-Vis-NIR diffuse reflectance spectrum shows that the reflectivity at 190 nm is 41.18,and the experimental band gap is Eg=5.96 eV,indicating that Sr₂B₅O₉（OH）·H₂O crystal may be used in the deep-ultraviolet region.