Niobates: High-Temperature And High-Pressure Synthesis And Luminescence Properties

Niobates with the crystal structure of compounds are very abundant that exhibit awide range of special physical properties, such as high dielectric constants and strongcatalytic activity. It is also a kind of important luminescence host material.White light emitting diode （LED） is a new semiconductor lighting source, andone of the key technologies is the fabrication of high effective phosphor. Niobiumoxides materials doped with rare earth ions have attracted most interesting because ofunique properties of light, electricity and magnet. Now, rare earths ions dopedniobates materials display an attractive prospect in practical application such as indisplay, lighting, opto-communication, laser device, X-ray sensor and so on. Theoptical properties of niobates which is researched not enough systematic andcomprehensive. So more researches are necessary for niobates single crystal, theyshould be studied significance in theory and practical application value.1. As you know, the physical properties of water can be changed greatly underhigh pressure supercritical water. The undissolved matter whose solubility is greatlyimproved which is benefical to the growth of single crystal. Increase pressure can getthe same effect by high temperature flux. Introduce the high pressure into this systemcan improve the size of the crystal.The large RE³⁺doped niobates whiskers can be obtained by high temperaturehigh pressure hydrothermal synthetic which combined with the flux method Thegeneral conditions for reactions are: heating temperature of400⁷00oC, reactiontime of2⁹6hours, pressure of50¹50MPa and mKOH：mNaOH=1:1,3:2,2:3. When mKOH：mNaOH=1:1, the alkali metal is applied as flux agents which don’t takepart in the reaction. We successfully synthesized large Dy³⁺doped GdNbO₄singlecrystals at650oC and110MPa for60h, the quantum yield of the material was⁶.86%. Eu³⁺doped GdNbO₄single crystals were synthesized at650oC and110MPafor36h, the quantum yield of the material was³1.2%. Dy³⁺doped YNbO₄singlecrystals were synthesized at700oC and110MPa for24h. the quantum yield of thematerial was²2.7%. The whole reaction process we consider the pressure,temperature, reaction and the influence of the alkali metal content on single crystalsynthesis and structure. Alkali metal acted as the flux which can not only acceleratethe reaction but also provide a suitable liquid environment for crystal growth.2. The growing interest in the fabrication and characterization of rare earthoxides （RE2O3, RE=La, Gd, Lu, Y） is related to the wide range of potentialapplications that can be foreseen in the realm of high-power lasers, due to their largetransparency, mechanical hardness, widely energy band gap. High pressuretechnology has made remarkable achievements; we have produced micrometer-sizedGd2O3single crysal by high temperature and high pressure hydrothermal method.How to get RE2O3single crystal, and the experimental period is short and low costwhich should be a huge challenge.3. Rare earth doped niobates synthesized by a high temperature solid process.The concentration of Eu³⁺and Tb³⁺should be adjust, Bi³⁺is a sensitizer. The finalproducts were characterized by means of X-ray powder diffraction （XRD）, fieldemission scanning electron microscope （SEM）, and photoluminescencespectrophotometer （PL）. The results showed that degree of Eu³⁺, Tb³⁺ion-emittingsignificant improvement was occurred when Bi³⁺doped. On the chromaticitycoordinate we can observe the white point.4. The distortion of the structure affected the spectrum. High pressurespectroscopy is considered as an effective tool for investigation of energetic structureof rare earth and transition metal ions in solids and interactions of localized electronswith lattice. In this contribution, the results of the pressure dependence of the energyof4fn-15d14fninterconfigurational transitions in Gd³⁺and intraconfigurational transitions in the Bi³⁺in selected crystals are summarized. The standard equipment isthe optical high pressure cell with diamond （DAC） or sapphire anvils. High pressurespectroscopy has been extensively applied for the investigation of3d-3d opticaltransitions in transition metal ions in solids. The d-f transition under pressure has beeninvestigated by Bi³⁺doped GdNbO₄synthesized under1.5GPa at1400oC for0.5hwhich obtained higher crystallinity. Compare to atmospheric pressure, high pressurecause spectral redshift.In conclusion, we have successfully synthesized the single crystals of Dy³⁺doped GdNbO₄, Dy³⁺-doped YNbO₄, Eu³⁺doped GdNbO₄and Gd2O3. The crystalstructures were resolved by single-crystal X-ray diffraction. Niobates synthesized byhigh temperature solid method, Bi³⁺is a sensitizer. On the chromaticity coordinate wecan observe the white point. The crystal growth mechanism and the pressure effect onthe crystal growth and structure distortion are discussed.In a word, our research is greatly enriched the synthesis method and expand theapplication of high pressure technology and flux method in the field of syntheticmaterials.