Structural Characterization of B‑Site Ordered Ba₂Ln_2/3TeO₆ (Ln = La, Pr, Nd, Sm, and Eu) Double Perovskites and Probing Its Luminescence as Eu³⁺ Phosphor Hosts

Ba₂Ln_2/3TeO₆ (Ln = La, Pr, Nd, Sm, and Eu) double perovskites were synthesized via solid-state ceramic route. Preliminary X-ray diffraction studies indicated a pseudocubic structure with lattice parameters ranging from 8.55 to 8.44 Å for the substitution of rare earths from La to Eu. Raman spectra show the frequency dependence of various Raman bands with respect to rare-earth substitution and exhibit a significant shift in peaks to higher wavenumber region, which was observed only for symmetric stretching modes of LnO₆ and TeO₆ octahedra. In accordance with observed number of bands and group theoretical predictions, the most likely symmetry of all compounds in the Ba₂Ln_2/3TeO₆ system was found to be monoclinic with <i>P</i>2₁<i>/n</i> space group. Rietveld refinement of the XRD patterns further confirmed the <i>P</i>2₁<i>/n</i> space group and also the 1:1 rock salt ordering of the B-site cations. Diffuse reflectance spectra of Ba₂Ln_2/3TeO₆ showed the optical bandgaps of these compounds between 3.9 and 4.8 eV, indicating the suitability as luminescent host material. The reduction in bandgap energy with lanthanide contraction of rare-earth ions is attributed to the widening of conduction band with octahedral tilting. Photoluminescence (PL) spectra and PL excitation spectra of Ba₂La_{2/3–<i>x</i>}Eu_<i>x</i>TeO₆ (<i>x</i> = 0.025, 0.05, 0.075, 0.1, 0.125, 0.15) were investigated and found to exhibit bright orange-red emission under UV excitation. Chromaticity coordinates closely resemble those of commercial red phosphor Sr₂Si₅N₈:Eu²⁺, which points toward the possible applicability of these new red phosphors in solid-state lighting industry. Finally, Judd–Ofelt intensity parameters Ω_λ (λ = 2 and 4) were calculated, which indicate that Eu³⁺ ions occupy the symmetric octahedral B-site of the Ba₂La_2/3TeO₆