Syntheses, Structures, And Properties Of Deep-UV Optical Crystal Materials Based On [MO₄]（M=Be,P,S） Tetrahedron

Deep-ultraviolet（deep-UV,λ<200 nm）optical crystals play a pivotal role in modern laser science and technology.Second harmonic generation（SHG）response and birefringence are key performance parameters of deep-UV optical crystals.Tetrahedral groups have attracted widespread attention due to their high ultraviolet light transmittance and wide band gaps.However,due to the high symmetry of tetrahedral groups,which leads to low polarizability and optical anisotropy,tetrahedral groups exhibit weak SHG response and small birefringence.Therefore,how to use tetrahedral groups for structural design and assembly is the key to achieving SHG response and functional tunable of birefringence.In this thesis,based on the structure-activity relationship between crystal structure and properties,the design,synthesis,and performance research of tetrahedral structured deep-ultraviolet optical crystal materials are carried out.The main research contents are as follows:1)Synthesis,structure,and properties of deep-ultraviolet phosphate BaNa₂[PO₃（OH）]₂nonlinear optical crystal.Using the introduction of hydroxyl groups to improve the amount of twist of the PO₄tetrahedron,increase polarity and optical anisotropy,and improve the SHG response and birefringence characteristics,a new deep-UV nonlinear optical metal phosphate BaNa₂[PO₃（OH）]₂was synthesized.It was crystallized in the acentric orthogonal space group Fdd2（No.43）,and its structure included a highly twisted[PO₃（OH）]group and a NaO₅triangular bipyramid.[PO₃（OH）]and NaO₅are alternately connected to form an_∞²[NaPO₃（OH）]alveolate layer.These alveolate layers are interconnected viaO-sharing to_∞²[Na₂PO₆（OH）₂]double-layers.These double-layers are further interlinked via Na–O–P bonds to construct a 3D structure,in which Ba atoms are filled between the double-layers.BaNa₂[PO₃（OH）]₂can realize a good balance of deep-UV nonlinear properties covering short UV absorption edge less than 190 nm,strong SHG response with the magnitude of～1.2×benchmark KH₂PO₄（KDP）,and the largest birefringence with the experimental value of 0.064@590±3 nm in reported alkaline/alkaline-earth metal phosphates producing the shortest phase-matching output down to the wavelength of 164 nm.At the same time,its thermal stability can reach 376°C.These research results show that deep-UV NLO crystal BaNa₂[PO₃（OH）]₂is a potential candidate.2)Syntheses,structures,and properties of optical crystals of deep-ultraviolet fluoroberyllophosphate Na₄Be₂PO₄F₅and KBe[PO₃（OH）]F.Based on the characteristics of small birefringence caused by the small optical anisotropy of tetrahedral groups,a promising birefringent waveplate material was constructed,and two new fluoroberyllium phosphate crystals Na₄Be₂PO₄F₅and KBe[PO₃（OH）]were designed and synthesized.Na₄B_e2PO₄F₅is crystallized in monoclinic space group C2/c（No.15）,and its structural feature is a zero-dimensional structure constructed of a completely new[Be₄P₂O₈F₁₀]hexameric isolated group.The[Be₄P₂O₈F₁₀]group is formed by connecting three types of isolated tetrahedrons BeO₂F₂,BeOF₃,and PO₄,and allF atoms are located at the terminal sites.The[Be₄P₂O₈F₁₀]group is surrounded by Na+cations to form a zero-dimensional structure.Na₄Be₂PO₄F₅is the first zero-dimensional poly-fluoroberyllophosphate.Its zero-dimensional structure is mainly due to the terminal group effect of F atoms.KBe[PO₃（OH）]F is crystallized in monoclinic space group P2₁/n（No.14）.Its structure is a new_∞¹[BePO₃（OH）F]one-dimensional chain constructed by connecting BeO₃F and PO₃（OH）groups,with adjacent chains filled with K⁺cations.Na₄Be₂PO₄F₅with deep-UV transmission below 190 nm,manifests more suitable birefringence of 0.003 at1064 nm for use as a promising deep-UV zero-order waveplate material compared to the deep-UV crystal KBe[PO₃（OH）]F,which shows remarkably-enlarged birefringence of 0.025 at 1064 nm.Further analysis reveals that the differences in their birefringence index are mainly attributed to the following two points:first,small optical anisotropy of tetrahedral units is prone to small birefringence;secondly,the zero-dimensional structure has a low group stacking density and is beneficial for small birefringence,which has been further confirmed by systematic research on alkali metal beryllium phosphate.3)Syntheses,structures,and properties of optical crystals of deep ultraviolet fluoroaluminosulfate M₂AlF₃SO₄（M=NH₄,K,Rb,Cs）and K₃Li₆AlF₄（SO₄）₄.A series of fluoroaluminum sulfates,namely M₂AlF₃SO₄（M=NH₄,K,Rb,Cs）and K₃Li₆AlF₄（SO₄）₄,were constructed by introducing Al–O groups and alkali metals with different cationic radii.They are all constructed from the same structural units SO₄and AlO₂F₄.M₂AlF₃SO₄（M=NH₄,K,Rb,Cs）are isomorphic,and they are all crystallized in the orthogonal space group Pbcn（No.60）,showing a new one-dimensional chain structure composed of_∞¹[AlF₄（SO₄）]chains,while K₃Li₆AlF₄（SO₄）₄is crystallized in the triclinic space group P-1（No.2）,which is a zero-dimensional structure formed by the connection of rare[AlS₂O₈F₄]isolated groups.The performance test results show that their UV light transmittance is lower than 190 nm,and they can reach the deep-UV light region.The birefringence of（NH₄）₂AlF₃SO₄,K₂AlF₃SO₄,Rb₂AlF₃SO₄,Cs₂AlF₃SO₄,and K₃Li₆AlF₄（SO₄）₄at1064 nm is 0.024,0.015,0.013,0.011,and 0.009,respectively,showing significant differences in optical birefringence characteristics.Analysis shows that their optical birefringence is not only related to the SO₄tetrahedron and AlO₂F₄tetragonal bipyramid anion groups,but also exhibits a strong impact of cations on birefringence,specifically manifested as optical birefringence decreases as the radius of the cations increases.This study shows that the combination of SO₄tetrahedron and AlO₂F₄tetragonal bipyramid can not only enrich structural chemistry,but also utilize the size effect of cations to regulate structural birefringence properties.