Samples of Gd₂O₃ : Tb(3 mol%) phosphor were obtained by sol-gel method followed by annealing at 800^oC and 1200^oC 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^oC under optical excitation showed a direct charge transition through the bandgap with E_g=2.56 eV. After elevated annealing temperature the concentration of Tb⁴⁺ ions decreases due to reduction to Tb³⁺. 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, Tb³⁺+ photoluminescence spectra, far infrared and Raman spectroscopy spectra, structure of the bandgape, distribution of Tb³⁺ and Tb⁴⁺ in cation sublattice.

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