Morphology-dependent crystallization and luminescence behavior of (Y, Eu)2O3 red phosphors

(Y_0.95Eu_0.05)₂O₃ red phosphor particles with three distinctive morphologies of submicron spheres (up to 180 nm), microflowers (up to 10 μm) and microplates (up to 50 × 10 μm) have been converted from their respective precursors autoclaved (100–180 °C, 12 h) from mixed solutions of the component nitrates and hexamethylenetetramine (CH₂)₆N₄]. The three types of precursors were found to have the approximate compositions M(OH)CO₃·H₂O for the sphere (M = Y and Eu), M₄O(OH)₉NO₃ for the flower and M₂(CO₃)₃·3H₂O for the plate, and their formation domains were defined. Both X-ray diffraction and photoluminescence analysis indicated that a calcination temperature of ?800 °C is needed to attain a homogeneous (Y_0.95Eu_0.05)₂O₃ solid solution and thus improved luminescence. Morphology-confined crystal growth of (Y_0.95Eu_0.05)₂O₃ was observed from the microplates, yielding a significantly higher exposure of the (4 0 0) facets at elevated temperature. The three types of phosphors exhibited a substantial morphology-dependent photoluminescence (PL)/photoluminescence excitation (PLE) behavior, but did not differ much in the positions of the PLE/PL bands or in the asymmetry factor I(⁵D₀ → ⁷F₂)/I(⁵D₀ → ⁷F₁)] of the luminescence. Upon UV excitation into the charge transfer band at ～240 nm the microplates showed the strongest red emission at ～613 nm (the ⁵D₀ → ⁷F₂ transition of Eu³⁺) at a calcination temperature of 1000 °C, whose intensity was ～2.49 and 1.57 times those of the flowers and spheres, respectively. Fluorescence decay analysis yielded similar lifetimes of ～1.5 ± 0.1 ms for the 613 nm emission of the three morphologies, suggesting that the differing luminescence was largely morphology-dependent, rather than defect-dependent.