Syntheses And Photoluminescence Properties Of Rare-earth Niobates And Tantalates

Owing to the growing importance of energy storage and environmentalfriendliness, much attention has been paid to the solid-state lightings （SSL） as thealternative to the existing lighting technologies. The white lighting emitting diodes（WLEDs） have many advantages, such as high energy efficiency, long life, goodreliability and excellent performance at low temperature. Generally, WLEDs areproduced by mixing red, green and blue （RGB） LEDs, or combining the blue LEDand yellow phosphor （Y,Gd）₃（Al,Ga）₅O₁₂:Ce³⁺or using an ultraviolet （UV）LED-stimulated RGB phosphors. By means of the three methods, WLEDs withhigh efficiency, high chromatic stability and tunable color temperature, can befabricated by UV-LED chips coated with RGB phosphors. However, the lack ofefficient red phosphors drastically hinders the development of the UV WLEDs.1. Pure and rare-earth-doped LaKNaTaO₅single crystals have been prepared inopen corundum crucible by adding additional water in the system. It is proved thatthe addition of water can maintain the equilibrium concentration of H₂O and O^2-inthe hydroxide melts and increase the solubility of the metal oxide. SEM and EDSresults of single crystals demonstrate the feasibility of the method. Compared withthe silver tube, corundum crucible is a simple and low-cost alternative to meet theidea of green chemistry. Furthermore, samples doped with Eu³⁺and Tb³⁺showstrong red and green PL respectively at room temperature and the PL will bestrengthened with the decrease of temperature. Besides, for samples doped withTb³⁺, the PL intensity reaches a maximum when the mole ratio of Tb³⁺is5%. And the absolute quantum efficiencies of Eu³⁺and Tb³⁺doped LaKNaTaO₅are2.1%and22.9%, respectively. The excellent PL properties of LaKNaTaO₅doped withEu³⁺and Tb³⁺make them prospective for practical applications of red and greencomponent of WLEDs.2. We have successfully synthesized Lu₃TaO₇, Lu₃NbO₇and Y₃TaO₇nanophosphors by the hydrothermal method. According to the statistic analysis ofTEM images, the average size is7.5nm,5.9nm and4.0nm, respectively.Furthermore, we investigated the photoluminescence properties of Eu³⁺-dopednanophosphors. Our results demonstrate they could be excellent red phosphorcandidate of WLEDs. The optimum doping concentration for Eu³⁺doped Lu₃TaO₇,Lu₃NbO₇and Y₃TaO₇nanophosphors is5%,10%and5%, with the quantum yield4.3%,2.5%and1.4%, respectively. In comparison with those of Lu3MO7:Eu³⁺（M=Ta, Nb） nanophosphors, the emission intensities of Lu3MO7:Eu³⁺/Sr2+（M=Ta,Nb） are enhanced.3. Novel red-emitting Eu³⁺-activated La_1/3NbO₃phosphors in the NUV regionhave been prepared by a solid-state method at high temperature. Eu³⁺-activatedLa_1/3NbO₃shows red emission peak at616nm when the excitation wavelength is394nm and the CIE chromaticity coordinates locates at （0.64,0.36）, which is veryclose to the standard red chromaticity for the NTSC system. We have exploredthree co-doping methods to extend the excitation bands in the NUV region, i.e. toco-doping with （1） Bi3+, for the6s→6p excitation of the bismuth ion can be used toharvest the NUV light;（2） Ca2+or Ba2+, by lowering the site symmetry of Eu³⁺andmaking the transition parity partially permitted and （3） Sm⁽3+, for the⁶H_5/2→⁴K_11/2transition of Sm⁽3+is in the NUV region ca404nm. The high absorption and strongexcitation bands of a family of Eu³⁺-activated La_1/3NbO₃phosphors in the range of370-410nm are very favorable properties for the applications as red component oftricolor NUV WLEDs.La_1/3NbO₃:Sm⁽3+phosphor was synthesized by solid state reaction at hightemperature and its photoluminescence properties were investigated. Its strongestexcitation band is centered at406nm, ascribed to⁶H_5/2→⁴K_11/2transition of Sm⁽3+,in the range of near-Ultraviolet region （NUV,365-410nm）. And its strongest emission peak is at596nm, attributed to4^G_5/2→⁶H_7/2transition of Sm⁽3+. As a result,La_1/3NbO₃:Sm³⁺phosphor shows great potential for future application as alternativered-emitting NUV-based white LEDs. The luminescence intensity can be stronglyenhanced by co-doping cation in the La-site, namely Sr²⁺, Ba²⁺and Bi³⁺.