Synthesis of NaLn(WO4)2 phosphors via a new phase-conversion protocol and investigation of up/down conversion photoluminescence

The two-dimensional (2D) crystallite morphology and low OH-/Ln(3+) molar ratio of Ln(2)(OH)(4)SO4 center dot 2H(2)O (Ln = lanthanide) make it an ideal precursor for materials synthesis via phase conversion, which was manifested in this work by the direct generation of well-defined NaLn(WO4)(2) phosphor particles via hydrothermal reaction with Na2WO4. Kinetics study showed that pure NaLn(WO4)(2) can be produced by reaction at 180 degrees C for similar to 24 h or at 200 degrees C for similar to 6 h under WO42-/Ln(3+) = 10 M ratio. Morphology analysis revealed that, though NaLn(WO4)(2) evolved via re-precipitation, the layered crystal structure and 2D crystallite morphology of the precursor could have templated the nucleation/growth of NaLn(WO4)(2), leading to uniform particles (similar to 4-5 mu m) of a unique microdisc-like morphology. Under 394 nm excitation, the Ln = La0.95Eu0.05 phosphor showed down-conversion luminescence having an absolute quantum yield of similar to 35.4%, a fluorescence lifetime of similar to 1.13 ms for its 616 nm main emission, and color coordinates of around (0.63, 0.37). Under 978 nm laser excitation, the Ln = La0.97Yb0.02Ho0.01 and Ln = La0.97Yb0.02Er0.01 phosphors exhibited the strongest up-conversion (UC) luminescence at similar to 660 nm (the F-5(5) -> I-5(8) transition of Ho3+) and 551 nm (the S-4(3/2) -> I-4(15/2) transition of Er3+), average fluorescence lifetimes of similar to 178.3 and 82.3 mu s for the above emissions, and chromaticity coordinates of around (0.62, 0.38) and (0.25, 0.72), respectively. The two UC phosphors were also analyzed to exhibit UC luminescence through a two-photon mechanism. (C) 2020 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.