New Fluoride Sulfate Deep-ultraviolet Nolinear Optical Materials

Nonlinear optical(NLO)crystal materials greatly expand the tunable range of the lasers via frequency conversion and promote the rapid development of laser technology and they have important applications in photochemistry,lithography,biomedicine and other fields.Traditionally,research on deep-ultraviolet(UV)NLO materials has mainly focused on systems such as borates,carbonates and phosphates.Recently,the discovery of sulfate deep-UV NLO materials has attracted much attention.In this paper,based on sulfate,a series of fluorinated sulfate NLO crystal materials were synthesized.This work provided a new idea for the the structural design and synthesis of these materials.The specific content is as follows:1.K₂Mn₃(SO₄)₃F₂·4H₂O and Rb₂Mn₃(SO₄)₃F₂·2H₂O:Synthesis and characterization of two fluorinated sulfates with pseudo KTi OPO₄(KTP)structure.KTP is one of the commercial NLO crystal materials with the most excellent comprehensive properties.Structurally,there are many known KTP-like structures and their derivatives due to its high structural tolerance.In this paper,two non-centrosymmetric compounds K₂Mn₃(SO₄)₃F₂·4H₂O and Rb₂Mn₃(SO₄)₃F₂·2H₂O were obtained by a simple aqueous solution method.The two compounds have similar layered structures,which are composed of Mn O₄F₂/Mn O₃F₂(H₂O)octahedron and SO₄ tetrahedron interlinked to form pseudo-KTP layered structures.The difference between the two structures is that there are two extra crystal water between the layers in the K₂Mn₃(SO₄)₃F₂·4H₂O structure to maintain the stability of the structure.In addition,the power second harmonic generation(SHG)test of show that the two compouds have SHG response.This work realizes the transformation of KTP in the structure.2.KMg SO₄F and KZn SO₄F:Synthesis and characterization of two novel KTP-like fluorinated sulfate deep-UV NLO materials.Although the comprehensive performance of KTP is very superior,it is a pity that its UV absorption edge can only reach 350 nm,which largely limits its application in the deep UV region.In the first work,we successfully modified the KTP structure,and obtained two cases of fluorinated sulfates with pseudo KTP structure.However,they don't pass light in the deep-UV region.In addition,both compounds contain crystalline water in their structures,which leads to their low thermal stability.In order to solve these problems,two cases of KTP-like structure fluorinated sulfate KMg SO₄F and KZn SO₄F were successfully obtained by hydrothermal method using the strategy of allovalent substitution.The two fluorinated sulfates belong to Pna2₁ space group and exhibit similar structure as KTP.Powder samples of KMg SO₄F and KZn SO₄F show that they have SHG response.The UV-Vis diffuse reflectance test shows that the absorption edges of the both componds can reach the deep ultraviolet region.Careful theoretical calculations suggest that the substitution of Ti O₆ octahedra in KTP by Mg O₄F₂ or Zn O₄F₂ octahedra largely enlarges the bandgaps of both sulfates from the microscopic point of view.This finding may provide a useful reference for the band-gap engineering of NLO materials,as well as for other functional materials,such as photovoltaic materials.3.Deep ultraviolet nonlinear optical material K₂Zn₃(SO₄)(HSO₄)₂F₄.By adjusting the reaction conditions of hydrothermal synthesis,we further explored the system of fluorinated sulfate and successfully obtained a new deep-UV NLO material K₂Zn₃(SO₄)(HSO₄)₂F₄.The space group of K₂Zn₃(SO₄)(HSO₄)₂F₄ belongs to Cmc2₁.Structurally,the Zn O₃F₃ and Zn O₄F₂ octahedra are interconnected with the SO₄ and SO₃(OH)tetrahedra to form a two-dimensional layered structure,and the valence state equilibrium between the layers is maintained by K⁺.The results of powder test shows that the SHG response of K₂Zn₃(SO₄)(HSO₄)₂F₄ is about 0.3 times that of KH₂PO₄(KDP).The single crystal transmission spectrum test shows that this material can achieve deep-UV transmission.First-principles calculations show that the optical properties are mainly determined by the S-O group,and the octahedrons of Zn O₃F₃ and Zn O₄F₂ have a certain effect on the bandgap.This work enriches the diversity of nonlinear optical materials in deep-UV NLO materials.