Structural Properties, Photoluminescence, and Judd-Ofelt Parameters of Eu3+- Doped CoNb2O6 Phosphor

Trivalent Eu-activated CoNb2O6 phosphors were fabricated using the molten salt method, which provides enhanced homogeneity and low sintering temperature. The ceramic samples were examined by spectral and structural analyses. In X-ray diffractions, the single phase of orthorhombic columbite type CoNb2O6 structure was obtained for 0.5-10 mol% Eu3+ doping concentrations, while a two theta peak shift towards the smaller angles occurred. SEM examinations show an irregular morphology and sub-micron grain sizes. In photoluminescence (PL) spectra, the phosphors showed typical Eu3+ emissions with the 5F0 → 7FJ (J=0, 1, 2, 3, 4) transitions, and high emission peaks were observed at the 5D0 → 7F2 transition. The photoluminescence of CoNb2O6:Eu3+ decreased over 5 mol% because of the concentration quenching. The energy transfer mechanism and critical distance of the phosphor are the dipole-dipole (d–d) interaction, and 15.70 Å, respectively. The spectral features of the phosphors were assessed by calculating the Judd-Ofelt intensity parameters (Ω2, Ω4) from the PL emission spectrum. The low Ω2 parameter values or/and the Ω4>Ω2 trend for CoNb2O6:Eu3+ phosphors were related to the less covalent or more ionic character of the Eu3+–O2˗ bond and the high local symmetry of the Eu3+ sites, while the high Ω4 parameter values may be ascribed to the decrease in the electron density in the ligands.