Anomalous behaviour of refractive index: a case study through photoluminescence properties of Eu³⁺-doped tungstate phosphor

In the present study, the investigation of excitation-dependent emissions of europium (Eu³⁺)-ion-doped bismuth tungstate phosphor (BiLaWO6) and relative effects is discussed in detail. Several findings from the diffuse reflectance spectra (DRS) indicate the electronic transitions related to charge transfer bands (CTB) and ligand-to-metal charge transfer (LMCT) processes. The photoluminescence (PL) spectra, observed under different excitation wavelengths, revealing distinct Eu³⁺ ion transitions (f - f intraconfigurational) and varying emission intensities. The decay profiles of Eu³⁺ luminescence displayed wavelength-dependent behaviour, with decay times decreasing with the increase of excitation wavelength. This behaviour is attributed to varying contributions of non-radiative (NR) energy transfer processes and direct excitation of Eu³⁺ ions. An indirect approach of calculating the Judd-Ofelt (J-O) parameters has been employed. The refractive index (RI), calculated indirectly from the emission intensity data, showed a non-monotonic variation with excitation wavelength, peaking at a specific wavelength. This indicates that Eu³⁺ ions can serve as effective probes for investigating material optical properties near the bandgap. The results from this work ascertain that, Eu³⁺ ion can not only be used as a spectroscopic probe but also to probe the behaviour of material and its variation of refractive index near the bandgap (E_g) of the material.