Qualitative Prediction and Quantitative Illustration of Eu3+ → Eu2+ Self-Reduction in Sr4Al14O25:Eu Phosphors Using Preferential Occupancy Site Theory (POST) for Tunable wLED and Ratiometric Temperature Sensing

Abstract A high-temperature solid-state method was employed to synthesize pure-phase Sr4Al14O25:Eu matrices and their phosphors under corresponding conditions, providing a robust experimental foundation for their promising applications in tunable white light-emitting diode (wLED) and ratiometric temperature sensing. The Preferential Occupancy Site Theory (POST) was employed to quantitatively calculate the preferential occupancy sites of Eu3+ and Eu2+ and to qualitatively predict the Eu3+ → Eu2+ self-reduction phenomenon in Sr4Al14O25. The calculation results are in perfect agreement with experimental facts, as confirmed by photoluminescence (PL) and X-ray photoelectron spectroscopy (XPS) analyses. Firstly, X-ray diffraction (XRD) characterization confirms that the synthesized phosphors possess a single-phase orthorhombic crystal structure with the Pmma space group. Secondly, the influence of Eu3+ and Eu2+ doping on the luminescent properties was then investigated via photoluminescence (PL) spectroscopy. The self-reduction phenomenon of Eu3+ to Eu2+ in Sr4Al14O25:Eu3+ was confirmed by comparing the PL spectra of samples synthesized under reducing atmosphere versus air atmosphere. The sample prepared in a reducing atmosphere exhibited important emission from Eu2+, while for the sample prepared in air, the characteristic emission peaks of Eu3+ and Eu2+ were observed, confirming the partial self-reduction of Eu3+ to Eu2+ in samples synthesized under air atmosphere. In addition, X-ray photoelectron spectroscopy (XPS) results demonstrate the coexistence of Eu3+ and Eu2+, confirming that partial self-reduction of Eu3+ to Eu2+ occurs within the Sr4Al14O25 matrix. Lastly, the coexistence of Eu3+ and Eu2+ in the Sr4Al14O25 crystal lattice, combined with its temperature-dependent luminescent properties, makes this material a promising candidate for ratiometric temperature sensing.