Controlled Preparation And Crystal Morphology Regulation Of Cryolite Structure Na_xK_3-xGaF₆

Compared with other matrix systems,fluorinated materials have many advantages,such as low phonon energy,wide light transmission range and so on.Na YF₄ is the most typical fluoride system.Na YF₄ has two different crystal forms.Under the same conditions,the luminescence intensity of hexagonal Na YF₄ is more than ten times that of cubic Na YF₄,but it is difficult to synthesize hexagonal Na YF₄,so it is urgent to find a more suitable new matrix.Because of the smaller ion radius of Sc³⁺,the matrix na3scf6 has attracted extensive attention.Cryolite structure compounds are expected to become new matrix materials with excellent properties,and also have monoclinic and cubic crystal forms.It is of great significance to study the new cryolite structure compound matrix materials and explore the relationship between crystal form and luminescent properties.In this paper,cubic K₃Ga F₆ matrix pure phase with cryolite structure was prepared,and the effects of reaction temperature and time on the morphology and particle size of the samples were discussed.The synthesis conditions of Na_2.5K_0.5Ga F₆ from cubic crystal to monoclinic crystal were found by isomorphic substitution.The luminescent properties of the phosphors with the same composition and different crystal forms were prepared and compared.The main achievements are as follows.1.Cubic cryolite compound K₃Ga F₆ was successfully synthesized by coprecipitation,hydrothermal and solvothermal methods.By adjusting and controlling the preparation method,raw material ratio,reaction time and temperature,surfactant,reaction solvent type and other experimental conditions,the relationship between K₃Ga F₆ morphology and particle size was explored.The average grain size ranges from 100 nm to a few microns,and the morphology is spherical,hexagonal,flower,octahedral and irregular block.2.Through isomorphic substitution,the proportion of the elements in the position of the monovalent cations in Na_xK_3-xGa F₆ was adjusted,and the critical composition point of the sample during the transformation from cubic crystal to monoclinic crystal was explored under different holding time and reaction temperature.In the range of 4～12 h and 140～200℃,with the increase of holding time and reaction temperature,the monoclinic phase is more easily formed in the samples with the same composition.3.The Na_2.5K_0.5Ga F₆:x Er³⁺Phosphor synthesized at 140℃is cubic crystal,and the phosphor synthesized at 200℃is monoclinic crystal.Under the excitation of 980 nm infrared light,the emission spectra of the two samples show typical Er³⁺transition,including two main bands of green and red,and the emission light of the two groups of samples is yellow.With the increase of Er³⁺doping concentration,the emission intensity of the two groups of samples first increases and then decreases.The optimum doping concentration of cubic and monoclinic samples is 1%and2%,respectively.The emission intensity of monoclinic Na_2.5K_0.5Ga F₆:x Er³⁺is about 30 times that of cubic.