Novel dysprosium-doped phosphate phosphor comprising alkali and alkaline earth metals as a versatile thermally stable luminescent material

In this work, a series of KNa_1-xDy_x Ba₂(PO₄)₂ (where x = 1.5–3.5 mol%, in steps of 0.5 mol%) phosphors were successfully prepared using a conventional high-temperature solid-state synthesis approach. The phase purity of the synthesized material was identified via powder XRD and subsequently verified by Rietveld refinement, which revealed a single-phase trigonal crystal structure with space group P3¯m1 (No. 164). Surface morphology was analysed by SEM, which showed that the samples consist of multiple block-like structures formed by loosely arranged aggregated particles, with an average particle size of 0.59633 ± 0.02981 µm. The FTIR spectra exhibited strong, distinct vibrational bands corresponding to phosphate (PO₄³⁻) groups, indicating the successful incorporation of Dy³⁺ ions without any disruption to the host matrix. Photoluminescence emission spectra recorded under 350 nm excitation show blue and yellow emission bands attributed to ⁴F_9/2 → ⁶H_15/2 (481 nm & 485 nm) and ⁴F_9/2→ ⁶H_13/2 (576 nm) transitions of Dy³⁺, respectively. According to Dexter's theory, concentration quenching was observed at 2.5 mol% of Dy³⁺, primarily due to dipole-dipole interactions. The DRS approach was employed to calculate the optical band gap, yielding values of 6.11 eV (for undoped sample) and 6.21 eV (for the optimised sample). Additionally, analysis of the nephelauxetic effect indicated that Dy³⁺ ions form covalent bonds with surrounding ligands in the host structure. The optical characteristics, based on CIE 1931 analysis, show that the impurity-optimized sample exhibits CIE chromaticity coordinates of (0.413060, 0.444039), a dominant emission wavelength (λ_d) of 574.1 nm (yellow colour), colour purity of 57.3 %, and a correlated colour temperature (CCT) of around 3721 K, supporting its suitability for warm-white LED applications. Temperature-dependent CIE results indicate that even at elevated temperatures, the emission wavelength remains stable, with the phosphor retaining 83 % of its initial emission intensity at 423 K. Furthermore, fluorescence intensity ratio (FIR)-based thermal sensing revealed a maximum relative sensitivity (S_R) of 0.96 % K⁻¹ observed at 298 K estimated over the 298–500 K range, demonstrating the material potential for optical thermometry applications.