Peculiarities of changes in the intensities of the main photoluminescence bands of Tb$^{3+}$ ions and their satellites in polycrystalline Gd$_2$O$_3$ : Tb(3 mol%)
Abstract:
Samples of Gd$_2$O$_3$ : Tb(3 mol%) phosphor were obtained by sol-gel method followed by annealing at 800$^\circ$C and 1200$^\circ$C in air. At high annealing temperature, the intensities of the main emission bands 484 and 541 nm increase, but the ratio of the intensities of these emission bands to their satellites 493 and 549 nm respectively decreases. Based on the analysis of X-ray diffractometry, emission spectra, far-infrared and Raman spectroscopy, as well as diffuse reflectance spectroscopy, we established: the increase in the crystallinity of the samples with a significant reduction of the lattice strain stress at elevated annealing temperatures, changes in the structure of the bandgap with degenerated acceptor and donor zones of impurities Tb4+ and Tb3+ respectively. The diffuse reflection spectra of the sample after annealing at 800$^\circ$C under optical excitation showed a direct charge transition through the bandgap with $E_g$ = 2.56 eV. After elevated annealing temperature the concentration of Tb$^{4+}$ ions decreases due to reduction to Tb$^{3+}$. As a result, at low excitation energies the degeneracy of the acceptor zone is still preserved and there is a direct transition of charges through the bandgap with $E_g$ = 2.55 eV. At high excitation energies the degeneracy of the acceptor zone is removed and there is a direct transition through the bandgap with $E_g$ = 3.39 eV. These effects are accompanied by a relatively large increase in the emission intensity of the satellites, especially at the 549 nm.
Keywords:Gd oxide, Tb$^{3+}$ + photoluminescence spectra, far infrared and Raman spectroscopy spectra, structure of the bandgape, distribution of Tb$^{3+}$ and Tb$^{4+}$ in cation sublattice.
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