Thermally stable photoluminescence of Eu3+ doped strontium pyrophosphate: red emitting phosphor with high color purity

Abstract

A series of Eu³-doped strontium pyrophosphate (Sr₂P₂O₇) samples was developed by a combustion synthesis process to investigate their photoluminescence (PL) properties. X-ray diffraction analysis was conducted to identify the creation of an alpha phase with an orthorhombic crystal structure. Bond formation of a pyrophosphate group in the structure of phosphor was recognized through FTIR spectra. The surface morphology of the particles was studied through SEM. The energy band gap of the undoped phosphor was calculated to be 5.04 eV through analysis of diffusion reflectance spectra (DRS), and it gradually changed with doping concentration. The phosphors were excited under NUV–blue light i.e., 396 nm and 466 nm, and displayed prominent red emission at a wavelength of 618 nm. The PL spectra of Eu³⁺ doped Sr₂P₂O₇ were examined for the dependency of PL intensity on doping ion concentration and excitation energy. The optimum PL intensity was obtained at 0.3% concentration of Eu³⁺, and intensity quenching was observed with a further increase in concentration. This occurred due to multipolar interaction, which was confirmed through the critical distance R_c value of 12.6 Å. Temperature-dependent PL was recorded under 396 nm excitation for an optimized phosphor (0.3% Eu³⁺). Reduction in PL intensity was observed at higher temperature, but 82% intensity persisted at 540 K with respect to the intensity attained at room temperature i.e., 300 K. The phosphor has high thermal stability, which is confirmed by the activation energy value of 0.206 eV. The PL emission decay lifetime was determined to be of the order of milliseconds for different concentrations of Eu³⁺. CIE chromaticity coordinates values were assessed from the PL spectra recorded at 396 nm excitation and the color purity was also determined for all the doped phosphors. The quantum efficiency of the ⁵D₀ level transition was calculated to be 95.5% to 85.5% for all doped samples. The experimental results were further compared with determined Judd–Ofelt parameters. CIE and CCT values for 0.3% Eu³⁺-doped phosphor were evaluated to be (0.638, 0.364) with color purity of 91.5%. The results obtained for the as-prepared Eu³⁺-doped Sr₂P₂O₇ phosphor are quite significant for application of the phosphor as a red-emitting phosphor in pc-WLEDs.