Synthesis,Structure And Properties Of Metal Silicate Deep Ultraviolet Nonlinear Optical Crystal

Deep-ultraviolet（Deep-UV）nonlinear-optical crystals are widely used in laser technology.The synthesis of new deep ultraviolet nonlinear optical crystals is currently a research hotspot.Metal silicate crystal materials show potential deep-UV nonlinear optical properties.In this study,Be O₄ tetrahedron with deep-UV light transmission capability and fluorine which is beneficial to pass through ultraviolet light were introduced into silicate system.By adopting high-temperature solid-state solution technology,Four deep-UV silicates optical crystal materials were designed and synthesized.Their structures and properties were analyzed.（1）Two deep-UV nonlinear-optical crystals M₂Be Si O₄（M=Li,Na）were synthesized.Li₂Be Si O₄ crystallizes in the orthorhombic non-centrosymmetric space group Pmn2₁（No.31）.In this structure,the interconnections of Si O₄ and Li O₄ tetrahedra form one-dimensional[Li Si O₅]_∞chains,and（Be/Li）O₄ tetrahedra polymerizes to form one-dimensional[（Be/Li）₂O₅]_∞chains.Two kinds of one-dimensional chains are alternately connected to form a two-dimensional[Li Be Si O₄]_∞layer in the ab plane.The two-dimensional layers are stacked along the c-axis to form three-dimensional structure of Li₂Be Si O₄.Na₂Be Si O₄ crystallizes in the orthorhombic non-centrosymmetric space group Pca2₁（No.29）.In this structure,two Si O₄and two Be O₄ tetrahedra are interlinked to form the Si₂Be₂O₁₂ group.The Si₂Be₂O₁₂ groups further forms a two-dimensional[Be₂Si₂O₄]∞layer with six-membered rings through the oxygen-sharing connection,and the layers by the vertice-shaing are further connected to form the three-dimensional structure of Na₂Be Si O₄.Na⁺cations are filled in the six-membered ring to maintain charge balance.The compound Li₂Be Si O₄and Na₂Be Si O₄exhibit short UV cut-off edge below 200 nm down to the deep-UV region,large nonlinear-optical response（Li₂Be Si O₄:0.7×KDP;Na₂Be Si O₄:1.1×KDP）,and good thermal stability（Li₂Be Si O₄:856°C;Na₂Be Si O₄:1156°C）.First-principles calculations show that the optical properties are mainly due to the contribution of Si O₄ and Be O₄ tetrahedra.The successful synthesis of Li₂Be Si O₄and Na₂Be Si O₄ provides a new research direction for designing novel deep ultraviolet nonlinear optical crystals.（2）A deep-UV nonlinear-optical crystal Na₁₀Be₄Si₄O₁₇ was synthesized.Na₁₀Be₄Si₄O₁₇crystallizes in hexagonal non-centrosymmetric space group P（?）3m（No.215）.In this structure,four Be O₄ tetrahedra are interconnected to form Be₄O₁₂ cluster with the O center.One Be₄O₁₂cluster are further connected to four surrounding Si O₄ tetrahedra to generate large Be₄Si₄O₁₇cluster.The isolated Be₄Si₄O₁₇ clusters are surrounded by Na⁺cations to form the zero dimensional structure of Na₁₀Be₄Si₄O₁₇.Na₁₀Be₄Si₄O₁₇ exhibits short UV cut-off edge below200 nm,moderate nonlinear-optical response（0.6×KDP）,and good thermal stability（1045°C）.First-principles calculation show that the optical properties are mainly due to the contribution of Si O₄ and Be O₄ tetrahedra.The successful synthesis of Na₁₀Be₄Si₄O₁₇ shows the flexible connection mode of the beryllium-silicon group,which provides a broader development space for the synthesis of deep ultraviolet beryllium silicate nonlinear optical crystals.（3）A deep-UV optical crystal Na Mg₂Si O₄F was synthesized.Na Mg₂Si O₄F crystallizes in the monoclinic center symmetric space group P2₁/c（No.14）.In this structure,the Mg O₄F₂octahedrons by shared-edges and shared-corners to form rhombic Mg₄O₁₀F₆ groups,the Mg₄O₁₀F₆ groups are interconnected to form two-dimensional[Mg₄O₁₀F₆]∞layers,the[Mg₄O₁₀F₆]∞layers are bridged by isolated Si O₄ tetrahedra to form a three-dimensional[Mg₂Si O₄F]_∞framework,and Na⁺cations are located framework to maintain charge balance.The structure exhibits short UV cut-off edge below 200 nm and good thermal stability（1024°C）.First-principles calculations show that the optical properties are mainly due to the contribution of Si O₄ and Be O₄ tetrahedra.