Effects of activator oxidation number and matrix composition on structure feature, microscopic morphology, and luminescence behavior of blue-emitting (Gd,Y)3Al5O12:Bi phosphors

摘要

Bismuth cation as an excellent activator for luminescent materials has attracted growing interest due to its abundant emission colors via varying matrix structures. However, the effect of valence conversion between trivalent Bi3+ and pentavalent Bi5+ on visible luminescence has not been reported so far. In this work, we interestingly found that 5-12.5 at.% Bi ion doped in (Gd,Y)3Al5O12 matrix generally existed in the oxidation state of +3, while an increase in its concentration induced coexisting valences of +3 and +5. Singly ionized oxygen vacancies are the primary defect form in garnets and the presence of Bi5+ would further compensate the oxygen defects. The activator valence and matrix composition impose great influence on the structure feature, microscopic morphology, and photoluminescence behavior of (Gd,Y)3Al5O12:Bi phosphors. Their lattice dimension gradually expands with increasing Bi ion concentration from 5 to 12.5 at.%, while more activator addition induces the presence of Bi5+ and lattice contract. The precipitation precursors obtained from chemical coprecipitation present hollow structures and their sizes obviously decrease with greater Bi addition, whilst their calcined garnet products become finer at first and then apparently coarsen with an increase in Bi ion concentration from 5 to 20 at.%. The (Gd,Y)3Al5O12:Bi phosphors exhibit blue-emitting spectral characteristics arising from 3P0 -> 1S0 transition of Bi3+ and the emergence of Bi5+ causes blue-shift excitation spectra and red-shift emission spectra. A higher Y3+/Gd3+ ratio in matrix composition increases the emission intensity, color purity, quantum efficiency, while shortens the fluorescence lifetime. The (Y0.875Bi0.125)3Al5O12 sample exhibits good thermal stability with a relatively high activation energy of similar to 0.43 eV.

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