Site selective substitution and its influence on photoluminescence properties of Sr0.8Li0.2Ti0.8Nb0.2O3:Eu3+ phosphors

Abstract

Perovskite red phosphors Sr_0.95−xEu_0.05Li_xTi_1−xNb_xO₃ (0 ≤ x ≤ 0.3), Sr_0.8−xLi_0.2Ti_0.8Nb_0.2O₃:xEu³⁺ (0 ≤ x ≤ 0.16) and Sr_0.8−x/2Li_0.2Ti_0.8−x/2Nb_0.2O₃:xEu³⁺ (0 ≤ x ≤ 0.16) were prepared by solid state reaction. The influence of increase of Eu³⁺ concentration and effect of altering substitution sites of Eu³⁺ ions from A-sites to simultaneously at A- and B-sites of the host lattice on the photoluminescence properties of the phosphors were investigated. The site selective substitution of Eu³⁺ ions is described with changes in cell volume obtained from X-ray diffraction patterns, Raman shifts and the decay curve profiles obtained from luminescence life time measurements. From the Raman spectra it is found that simultaneous substitution of Eu³⁺ ions at A- and B-sites increases the intensity of first order TO₂, TO₄ and LO₄ vibrational modes at 184, 541 and 811 cm⁻¹ and the first order R-point vibrations at 147, 229 and 449 cm⁻¹ significantly, indicating excess defect concentration. The single exponential luminescence decay curves observed for Sr_0.8−xLi_0.2Ti_0.8Nb_0.2O₃:xEu³⁺ phosphors reveal A-sites occupancy and the double exponential nature of decay curves of Sr_0.8−x/2Li_0.2Ti_0.8−x/2Nb_0.2O₃:xEu³⁺ phosphors confirm the simultaneous occupancy of europium at A- and B-sites. Interestingly we observed a sharp increase in asymmetric ratio between the electric and magnetic dipole transitions ⁵D₀ → ⁷F₂ and ⁵D₀ → ⁷F₁, from 2.09 to 4.09, on doubling the substitution sites of Eu³⁺ resulting in enhanced color purity. It is found that simultaneous substitution increases the intensity of red emission at 615 nm, under optimum doping conditions, by 1.46 times. Further, from Judd–Ofelt analysis, J–O intensity parameter Ω₂ is found to increase from 3.67 × 10⁻²⁰ to 4.80 × 10⁻²⁰ cm², on altering substitution of Eu³⁺ from A-sites to A- and B-sites simultaneously, substantiating the increase of local asymmetry in the lattice. The radiative and non-radiative transition rates, radiative life time, branching ratios and stimulated emission cross sections of all these phosphors are also reported. The color coordinates are described with CIE chromaticity diagram. Color purity and color temperature of these phosphors are also described in detail.