Synthesis and characterization of Sm³⁺ activated La_1−xGd_xPO₄ phosphors for white LEDs applications

A series of orange-red light emitting Sm³⁺ activated La_1−xGd_xPO₄ (0.00 ≤ × ≤ 1.00) phosphors were synthesized by the solid-state method. The structural parameters were confirmed by the Rietveld refinement method based on powder X-Ray diffraction (XRD) analysis. All the compounds crystallized in the monazite monoclinic structure with space group P 1 2₁/n 1 (no. 14). The photoluminescence spectra of La_0.95−xGd_xPO₄:Sm³⁺ phosphors were measured at the excitation wavelength of 400 nm, exhibited characteristic emission peaks for Sm³⁺ at 560, 597, and 643 nm. The purely magnetic dipole allowed transition (⁴G_5/2 → ⁶H_5/2) at 560 nm and partly magnetic dipole transition (⁴G_5/2 → ⁶H_7/2) at 597 nm, responsible for orange-red light, dominated the emission spectra. In contrast to the magnetic dipole transitions, the electric dipole transition (⁴G_5/2 → ⁶H_9/2) was found to be relatively less intense confirming high symmetrical crystal environment around Sm³⁺ in the host lattice. However, with subsequent substitution of Gd³⁺ at the lanthanide site in the host lattice, the crystal field suffered distortion and thus, influenced the photometric properties. From experimental results, it was evident that these phosphors have suitable Commission International de l’Eclairage (CIE), color correlated temperature (CCT) parameters, appreciable lifetime, and excellent color purity with respect to other reported rare earth ion doped orange-red phosphors. Further, these results could help in the improvisation of their use in optoelectronics especially white LEDs, photovoltaic cells and other strategic applications.