Investigation of luminescence properties and ratiometric thermometry through yellow-to-blue Dy$^{3+}$ emission in Ca$_3$La$_7$(SiO$_4$)$_5$(PO$_4$)O$_2$ apatite

Dy$^{3+}$-doped Ca$_3$La$_7$(SiO$_4$)$_5$(PO$_4$)O$_2$ (CLSPO) phosphors were synthesized via 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 (P6$_3$/m) with refined lattice parameters of a = b = 9.604(3) Å, c = 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$^{3+}$ emissions, with two blue bands (B$_1$: 468 nm, B$_2$: 479 nm) and two yellow bands (Y$_1$: 543 nm, Y$_2$: 575 nm). The yellow-to-blue (Y/B) intensity ratio displayed a strong temperature dependence, establishing CLSPO:Dy$^{3+}$ as a promising candidate for luminescence-based thermometry. The optimal Dy$^{3+}$ 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. ... mehrThe 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$^{3+}$ concentration (from 658 μs at 0.5 at% Dy$^{3+}$ to 252 μs at 10 at%). The fluorescence intensity ratio (FIR) method for optical thermometry revealed an absolute sensitivity (S$_a$) of 3.27 10$^{−4}$ K$^{−1}$ at 298 K, while the repeatability (R) of the Y/B 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$^{3+}$ phosphors as highly efficient, thermally stable, and optically robust materials for next-generation temperature sensors, solid-state lighting, and advanced photonic applications.