UC/DC luminescence of Ho3+ doped pyrochlore structured La2(1-x)Yb2xTiO5 phosphor synthesized by sol-gel method

Ho3+ doped La2(1-x)Yb2xTiO5 samples were prepared by sol-gel method. Pyrochlore structured powders were obtained under the annealing of the as-prepared samples at 1100 degrees C for 3 h in the air. All the samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM)and photoluminescent spectroscopy. When x= 0, we obtained La2TiO5 phosphors. Monitored at 546 nm, the excitation spectrum of Ho3+ activated La2TiO5 phosphors consisted of a broad band originating from the charge-transfer band (CTB)between Ho3+ ions and the nearest neighboring O2- ions at 250-280 nm and four sharp bands associated to f-f transitions of Ho3+ ions at 350-500 nm. Under the blue light excitation at 461 nm, Ho3+ ion in La2TiO5 emitted an intense green emission band at 546 nm due to the transition of S-5(2), F-5(4) -> I-5(8). Two intense bands were observed at 490 nm (blue, F-5(3) -> I-8) and 663 nm (red, F-5(5) -> I-5(8))in the up-conversion (UC)spectrum under 980 nm NIR laser excitation in La2TiO5 phosphors activated with Ho3+ alone, whereas they were much different from the down-conversion (DC)luminescent samples. When x= 7.5 mol.%-17.5 mol.%, the UC emission spectra of them dominated at 546 nm (green, S-5(2),F-5(4) -> I-5(8))and enhanced significantly with the dose of Yb3+ ion. The decay curve also confirmed that the transitions of F-5(4) -> I-5(8) and S-5(2) -> I-5(8) were merged into one emission band at 546 nm.