Multifunctional Role of Eu ³⁺ in NaSrBiTeO ₆ Double Perovskites for Solid-State Lighting, Temperature Sensing, and Latent Fingerprint Applications

Strong red-emitting cubic NaSrBiTeO6:Eu3+ double-perovskite phosphors with different Eu3+ concentrations were synthesized using a solid-state diffusion method at 850 °C. Rietveld refinement of the XRD patterns confirmed that the incorporation of Eu3+ did not modify the crystal structure framework. Characteristic vibrational modes were observed using FTIR and Raman spectroscopy, whereas SEM-EDX analysis revealed irregular microcrystals and a uniform distribution of dopants. An internal quantum efficiency (IQE) of 16.44% was achieved for 7% Eu3+-doped NaSrBiTeO6 when excited at 465 nm. Temperature-dependent photoluminescence studies confirmed the excellent thermal stability of the phosphor, with 90% of its emission being retained even at 353 K. The temperature sensitivity parameters (Sa = 0.0466 K–1, Sr = 0.4535%K–1) determined using the fluorescence intensity ratio highlight the suitability of the developed phosphors for optical thermometry. The phosphor’s color purity of 94.36% and correlated color temperature of 2174 K validate its suitability for warm pc-WLEDs. In addition to lighting, NaSrBiTeO6:Eu3+ powders exhibited superior performance in latent fingerprint detection when subjected to 365 nm UV illumination.