Ho³⁺-doped SrNb₂O₆ phosphors for enhanced green luminescence and lifetime-based optical thermometry

Ho³⁺-doped SrNb₂O₆ phosphors were successfully synthesized via a high-temperature solid-state reaction method and investigated for structural, morphological, luminescence, thermal stability, colorimetric, and optical thermometric properties. X-ray diffraction confirmed the formation of a single-phase monoclinic structure with space group P2₁/c. Williamson–Hall analysis revealed defect-induced lattice strain of 0.26 × 10⁻³–1.66 × 10⁻³ with crystallite sizes of 92.44–149.41 nm, while SEM analysis showed agglomerated polyhedral particles with an average particle size of 4.19 ± 0.14 μm. Diffuse reflectance spectra exhibited characteristic Ho³⁺ absorption transitions with optical band-gap values of 3.87–3.90 eV and a strong absorption band near 451 nm, supporting efficient blue-light excitation. Excitation–emission contour mapping identified the optimum excitation and emission wavelengths at 452 and 547 nm, respectively. Under 452 nm excitation, intense green emission originating from the (⁵F₄, ⁵S₂) → ⁵I₈ transition of Ho³⁺ ions was observed with maximum intensity at 3 mol% Ho³⁺ concentration. The phosphors exhibited excellent colour purity of 99.3–100% and correlated colour temperature values of 6287–6492 K, indicating highly saturated cool-green emission. Concentration quenching occurred through dipole–dipole interaction with a critical distance of 19.6 Å. The optimized phosphor retained 70.44% of its initial emission intensity at 423 K with an activation energy of 0.2641 eV. Furthermore, lifetime-based optical thermometry demonstrated maximum absolute and relative sensitivities of 5.1 × 10⁻⁵ ms K⁻¹ at 373 K and 0.30% K⁻¹ at 423 K, respectively, highlighting the potential of Ho³⁺-doped SrNb₂O₆ phosphors for advanced green-emitting photonic devices and optical temperature-sensing applications.