Study Of Birefringent Crystals Of Tin(Ⅱ) Borates And Phosphates

Birefringence is one of key performance for optical functional crystals.In recent years,the cations containing lone pair electrons(Sn²⁺,Pb²⁺,Sb³⁺,Bi³⁺,Te⁴⁺,etc.),which are prone to form asymmetric motifs with ligand anions(O^2-,S^2-,etc.),i.e.the stereochemically active lone pair electron（SCALP）effect,are conducive to strong optical property gain and therefore become a current research hotspot for exploring birefringent crystals.In this paper,borates and phosphates containing Sn²⁺are explored as a research system,and UV-optical crystals with excellent properties are structurally designed and synthesised artificially with birefringent crystals.After a long period of research,three cases of birefringent crystals containing Sn²⁺and one case of alkali metal borate chloride UV-optical crystals were obtained by high-temperature solution method and hydrothermal method,and their properties were characterized.The main points are as follows:（1）A borate birefringent crystal containing Sn²⁺:Sn₃B₁₀O₁₇Cl₂A new tin boronate chloride,Sn₃B₁₀O₁₇Cl₂,was synthesized using a high-temperature melt method in a vacuum environment,crystallizing in the orthorhombic Pbcn（No.60）space group and characterizing by a three-dimensional framework with pores composed of the basic structural unit B₅O₁₁ interconnected by shared oxygen atoms,Cl and Sn atoms located inside and at the edges of the pores to balance the charge.In addition,optical property measurements show that Sn₃B₁₀O₁₇Cl₂ has a UV cut-off edge of approximately 280 nm.Theoretical calculations show a large optical anisotropy and a birefringence value of 0.125 at 546 nm,while the first principles calculations were used to analyze the relationship between structure and properties.（2）Phosphate birefringent crystals containing Sn²⁺:Sn（Ⅱ）Sn（Ⅳ）（PO₄）₂ and Sr Sn（PO₄）PO₂（OH）₂Two new tin phosphates,Sn（Ⅱ）Sn（Ⅳ）（PO₄）₂ and Sr Sn（PO₄）PO₂（OH）₂,were synthesized by high-temperature melt and hydrothermal methods under vacuum.they crystallized in the monoclinic system P2₁/c（No.14）space group and triclinic system P1（No.2）space group,respectively.Sn（Ⅱ）Sn（Ⅳ）（PO₄）₂ exhibits a three-dimensional framework composed of PO₄,Sn（Ⅳ）O₆ and Sn（Ⅱ）O₃ interconnected by shared oxygen atoms.Sr Sn（PO₄）PO₂（OH）₂ exhibits a three-dimensional framework composed of PO₂（OH）₂,PO₄,Sr O₈ and Sn O₄ interconnected by shared oxygen atoms.UV-Vis-NIR diffuse reflectance spectroscopy shows that Sn（Ⅱ）Sn（Ⅳ）（PO₄）₂ and Sr Sn（PO₄）PO₂（OH）₂ have UV cut-off edges of 262 nm and 263 nm respectively.Theoretical studies have shown that the birefringence is enhanced by the introduction of stereochemically active Sn²⁺into the metal phosphates,Sn（Ⅱ）Sn（Ⅳ）（PO₄）₂and Sr Sn（PO₄）PO₂（OH）₂ have birefringence values of 0.048 and 0.080 at 1064 nm,respectively.（3）An alkali metal borate chloride:Li₂B₅O₈ClA new alkali metal borate chloride,Li₂B₅O₈Cl,crystallized in the orthorhombic Pbcn（No.60）space group,was synthesized by a high-temperature melt method in a closed system.The anionic framework of this compound is composed of the basic structural unit B₅O₁₀interconnected to form a three-dimensional mesh,and interestingly,two three-dimensional meshes interpenetrate to form the final anionic framework,enriching the structural diversity of the B-O conformation.Furthermore,the first principles calculations indicate that the compound has a moderate birefringence,with an estimated birefringence of 0.030 at 1064 nm.