Exploration Of Deep Ultraviolet Nonlinear Optical Crystals Of Alkali Metal Hydrated Borates

Ultraviolet nonlinear optical?UV NLO?crystal has attracted much attention due to its important applications in optoelectronic technology.Over the years,the search for novel and excellent ultraviolet NLO crystals has never stopped.However,many of the NLO materials found so far have been unable to meet the needs of practical applications due to their own defects.Therefore,the development of new UV NLO materials are still in progress.So far,although the detection system of ultraviolet NLO materials involves borates,phosphates,carbonates and nitrates,a large number of theoretical and experimental studies have shown that borates are still the most promising candidate materials for ultraviolet harmonic generation.It is well known that boron is an oxyphilic element,which can be combined by sp² hybrid orbital or sp³ hybrid orbital to form a boron-oxygen plane triangle[BO₃]or tetrahedron[BO₄].These BO₃ and BO₄ units can be composed of different B_xO_y groups,forming isolated or infinite clusters,chains,sheets and frameworks by sharing oxygen atoms.It is worth noting that these different B_xO_y groups are considered to be the main factors affecting its optical properties.In addition,the introduction of alkali or alkaline earth metals can effectively increase the band gap of the compounds.After a lot of literature investigation,we found that hydrated borate crystals have a large space for exploration,there are two reasons:?1?the introduction of H atom can produce hydrogen bond,which is easy to distort the structure of boron and oxygen,and then produce the asymmetric structure;?2?H atom can form hydroxyl group with O atom or H₂O with O atom through flexible transformation of connection sites.These factors greatly enriched the structure of hydrated borates and laid a theoretical foundation for the design and synthesis of nonlinear optical crystals of hydrated borates.Due to the limited time,we only explored the deep ultraviolet nonlinear optical crystals of alkali metal hydrated borates.We successfully synthesized 4 deep ultraviolet optical crystals of hydrated borates by using ethanol solvothermal method,namelythecentrosymmetricLiRbB₅O₈?OH??LRB?,Rb₂B₆O₉?OH?₂,Cs₂B₁₀O₁₄?OH?₄·H₂O and the noncentrosymmetric K₂B₅O₈?OH?·2H₂O?K2B5?.Among them,K2B5 was reported by Marezio et al.in 1969^[1]and Liu Zhihong et al.in 2006^[2],respectively,but its nonlinear optical properties and other physical properties were not fully studied and characterized by them,while the other three centric crystals are reported for the first time by us.We have carried out single crystal structure determination,thermal analysis,UV-vis-NIR diffuse reflectance spectrum test,infrared spectrum test,theoretical calculation for 4 kinds of crystals and so on.K2B5 has also been subjected to laser damage threshold measurement and powder frequency multiplication coefficient measurement.In terms of structure,it is worth mentioning that K2B5 and LRB share the same basic unit structure block B₅O₁₀?OH?,but they have different H atomic connection sites and different connection modes between them.In addition to the size effect of cations,compounds LRB and K2B5crystallize in P2₁/n centrosymmetric space group and noncentrosymmetric space group Pna2₁,respectively.The UV-vis-NIR diffuse reflectance showed that they had short ultraviolet cutoff edge?<200 nm?,and the theoretical band gap calculation also showed that they all had wide band gaps.It is worth noting that compound K2B5 had high laser damage threshold and moderate second harmonic?SHG?intensity?0.84×KDP?.