Effects of Flux and Ta⁵⁺ Substitution on the Photoluminescence of Lu(Nb,Ta)O₄:Eu³⁺ Phosphors

융제 및 Ta⁵⁺ 치환이 Lu(Nb,Ta)O₄:Eu³⁺ 형광체의 발광 특성에 미치는 영향

$Lu(Nb,Ta)O_4:Eu^{3+}$ powders are synthesized by a solid-state reaction process using LiCl and $Li_2SO_4$ fluxes. The photoluminescence (PL) excitation spectra of the synthesized powders consist of broad bands at approximately 270 nm and sharp peaks in the near ultraviolet region, which are assigned to the $Nb^{5+}-O^{2-}$ charge transfer of $[NbO_4]^{3-}$ niobates and the f-f transition of $Eu^{3+}$, respectively. The PL emission spectra exhibit red peaks assigned to the $^5D_0{\rightarrow}^7F_J$ transitions of $Eu^{3+}$. The strongest peak is obtained at 614 nm ($^5D_0{\rightarrow}^7F_2$), indicating that the $Eu^{3+}$ ions are incorporated into the $Lu^{3+}$ asymmetric sites. The addition of fluxes causes the increase in emission intensity, and $Li_2SO_4$ flux is more effective for enhancement in emission intensity than is LiCl flux. The substitution of $Ta^{5+}$ for $Nb^{5+}$ results in an increase or decrease in the emission intensity of $LuNb_{1-x}Ta_xO_4:Eu^{3+}$ powders, depending on amount and kind of flux. The findings are explained using particle morphology, modification of the $[NbO_4]^{3-}$ structure, formation of substructure of $LuTaO_4$, and change in the crystal field surrounding the $Eu^{3+}$ ions.