Investigation of luminescence properties and ratiometric thermometry through yellow-to-blue Dy emission in CaLa(SiO)(PO)O apatite.

Dy-doped CaLa(SiO)(PO)O (CLSPO) phosphors were synthesized 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 (6/) with refined lattice parameters of = = 9.604(3) Å, = 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 Dy emissions, with two blue bands ( : 468 nm, : 479 nm) and two yellow bands ( : 543 nm, : 575 nm). The yellow-to-blue (/) intensity ratio displayed a strong temperature dependence, establishing CLSPO:Dy as a promising candidate for luminescence-based thermometry. The optimal Dy 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 Dy concentration (from 658 μs at 0.5 at% Dy to 252 μs at 10 at%). The fluorescence intensity ratio (FIR) method for optical thermometry revealed an absolute sensitivity ( ) of 3.27 10 K at 298 K, while the repeatability () of the / 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:Dy phosphors as highly efficient, thermally stable, and optically robust materials for next-generation temperature sensors, solid-state lighting, and advanced photonic applications.