Investigation of luminescence properties and ratiometric thermometry through yellow-to-blue Dy³⁺ emission in Ca₃La₇(SiO₄)₅(PO₄)O₂ apatite

Dy³⁺-doped Ca₃La₇(SiO₄)₅(PO₄)O₂ (CLSPO) phosphors were synthesized <i>via</i> a solid-state reaction method and characterized for their structural, optical, and thermometric properties. X-ray diffraction (XRD) and Rietveld refinement confirmed a hexagonal apatite-type structure (<i>P</i>6₃/<i>m</i>) with refined lattice parameters of <i>a</i> = <i>b</i> = 9.604(3) Å, <i>c</i> = 7.103(1) Å. First-principles calculations for the undoped crystal revealed a direct bandgap of 4.08 eV, confirming CLSPO as a suitable host material for luminescent applications. Photoluminescence spectra exhibited characteristic Dy³⁺ emissions, with two blue bands (<i>B</i> ₁: 468 nm, <i>B</i> ₂: 479 nm) and two yellow bands (<i>Y</i> ₁: 543 nm, <i>Y</i> ₂: 575 nm). The yellow-to-blue (<i>Y</i>/<i>B</i>) intensity ratio displayed a strong temperature dependence, establishing CLSPO:Dy³⁺ as a promising candidate for luminescence-based thermometry. The optimal Dy³⁺ doping concentration was determined to be 3 at%, beyond which concentration quenching effects were observed. Photoluminescence studies further demonstrated that electric dipole-dipole interactions govern the dominant energy transfer mechanism, as evidenced by concentration-dependent quenching behavior. The absolute photoluminescence quantum yield (PLQY) was 5.7%, and Arrhenius analysis determined an activation energy of 0.11 eV. The decay time decreases with increasing Dy³⁺ concentration (from 658 μs at 0.5 at% Dy³⁺ to 252 μs at 10 at%). The fluorescence intensity ratio (FIR) method for optical thermometry revealed an absolute sensitivity (<i>S</i> _a) of 3.27 10^-4 K^-1 at 298 K, while the repeatability (<i>R</i>) of the <i>Y</i>/<i>B</i> ratio exhibited a reproducibility of 95.88% at 298 K, ensuring consistent and reliable temperature sensing performance. Furthermore, the luminescence remained stable over three hours of multiple heating-cooling cycles (298-523 K), confirming excellent photostability and reversibility. These results establish CLSPO:Dy³⁺ phosphors as highly efficient, thermally stable, and optically robust materials for next-generation temperature sensors, solid-state lighting, and advanced photonic applications.