Phase evolution, structure, and up-/down-conversion luminescence of Li6CaLa2Nb2O12:Yb3+/RE3+ phosphors (RE = Ho, Er, Tm)
Garnet-type Li 6 Ca(La 0.97 Yb 0.02 RE 0.01 ) 2 Nb 2 O 12 (RE = Ho, Er, Tm) new phosphors were successfully synthesized via solid reaction at 900°C for 5 hours, whose course of phase evolution, macroscopic/local crystal structure and up-/down-conversion (UC/DC) photoluminescence were clarified. Mechanistic study and materials characterization were attained via XRD, Rietveld refinement, DTA/TG, electron microscopy (FE-SEM/TEM), and Raman/reflectance/fluorescence spectroscopies. The phosphors were shown to exhibit UC luminescence dominated by a ~ 553 nm green band ( 5 F 4 / 5 S 2 → 5 I 8 transition) for Ho 3+ , a ~ 568 nm green band ( 4 S 3/2 → 4 I 15/2 transition) for Er 3+ and a ~ 806 nm near-infrared band ( 3 H 4 → 3 H 6 transition) for Tm 3+ under 978 nm laser excitation, with CIE chromaticity coordinates of around (0.31, 0.68), (0.38, 0.60) and (0.17, 0.24), respectively. Analysis of the pump-power dependence of UC intensity indicated that all the emissions involve a two-photon mechanism except for the ~ 486 nm blue emission of Tm 3+ ( 1 G 4 → 3 H 6 ), which requires a three-photon process. The DC luminescence of these phosphors is featured by dominant bands at ~ 553 nm for Ho 3+ (green, 5 F 4 / 5 S 2 → 5 I 8 transition), ~568 nm for Er 3+ (green, 4 S 3/2 → 4 I 15/2 transition) and ~ 464 nm for Tm 3+ (blue, 1 D 2 → 3 F 4 transition). The UC and DC properties were also comparatively discussed.
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