Eu³⁺ ion concentration induced 3D luminescence properties of novel red-emitting Ba₄La₆(SiO₄)O:Eu³⁺ oxyapatite phosphors for versatile applications

A series of Eu³⁺ ions activated Ba₄La₆(SiO₄)₆O (BLSO:Eu³⁺) phosphors were synthesized by a modified citrate sol-gel process. The structural properties of these phosphors were explored by means of X-ray diffraction (XRD) and Fourier transform infrared spectroscopy. The XRD patterns confirmed their oxyapatite structure with the space group of P6₃/m after annealing at 1400 °C. The scanning electron microscope image exhibited the irregular morphology of BLSO:Eu³⁺ particles and the elemental mapping confirmed that the Eu³⁺ ions were distributed homogeneously on the La³⁺ ion sites. Photoluminescence (PL) excitation spectra of BLSO:Eu³⁺ exhibited the charge transfer band (CTB) and intense f-f transitions of Eu³⁺ ions in the violet and blue wavelength regions. The CTB intensity decreased and the f-f transition of Eu³⁺ ions increased with increasing the Eu³⁺ ion concentration due to the presence of defects in the 4f and 6h sites of the BLSO host lattice. This feature facilitates the strong absorption in the near-ultraviolet (NUV) region, which is useful for high color rendering index NUV based white light-emitting diodes for display and lighting applications. The PL spectra displayed intense red emission (⁵D₀ → ⁷F₂) along with considerable orange emission (⁵D₀ → ⁷F₁) with good asymmetry ratios and chromaticity coordinates, and exhibited better emission performance than that of commercial Y₂O₃:Eu³⁺ phosphors. The three-dimensional PL spectra revealed their strong emission characteristics under UV, NUV and visible excitation wavelengths. The cathodoluminescence properties were also similar to the PL results, confirming that the BLSO:Eu³⁺ phosphors emit stable red emission under different excitation sources as compared to the commercial Y₂O₃:Eu³⁺ phosphors.