Amphotericity-spectroscopy correlations in Eu doped sodium bismuth titanate (Na_0.5Bi_0.5TiO₃)

Optical characteristics of a doped material depend on the site wherein the dopant resides. In mixed A-site perovskites, the dopant has several possibilities. This, in turn, correlates with the emission spectrum of the doped material. Here we choose Eu³⁺ as the dopant since it is a well-known and efficient red-luminophore. Sodium bismuth titanate (NBT-Na_0.5Bi_0.5TiO₃) is selected as a host due to its ability to accommodate Eu over large concentration windows while maintaining optical activity. For compositions x ≥ 0.04, we show that Eu_x:NBT has amphoteric behavior (i.e., simultaneous dopant substitution in two competing sites). Eu³⁺ occupancy in Bi and Ti-sites are quantitatively estimated using Rietveld refinement. Also, the appearance of A_1g breathing vibrational mode (830–860 cm⁻¹) in Raman spectra shows the presence of Eu³⁺ on the Ti-site. Photoluminescence (PL) emission shows sharp and intense bands at ∼592 nm, ∼615 nm, and ∼687 nm, and one weak band at ∼652 nm, corresponding to f–f transitions of Eu³⁺. To evaluate the optical characteristics with varying site-substitution of Eu³⁺; Judd–Ofelt parameters and radiative properties are calculated for the first time in this mixed A-site titanate system. From the values of asymmetric ratios; Eu³⁺ ion is shown to occupy the non-centrosymmetrical sites (Bi-sites) of NBT. However, at x = 0.04, Ω₂ < Ω₄, which indicates long-range effects and hence Eu occupancy in centrosymmetric sites (Ti-sites). This study opens up avenues for further exploration of correlations between dopant-concentration, amphotericity, and spectrochemical response in other systems. The amphoteric behavior of dopants can tune emission intensities and produce multi-emission colors using a single-dopant. This would have implications for luminescence-based applications.