Structural Characterization of B-Site Ordered Ba2Ln2/3TeO6 (Ln = La, Pr, Nd, Sm, and Eu) Double Perovskites and Probing Its Luminescence as Eu3+ Phosphor Hosts.

Abstract

Ba2Ln2/3TeO6 (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 Ba2Ln2/3TeO6 system was found to be monoclinic with P21 /n space group. Rietveld refinement of the XRD patterns further confirmed the P21 /n space group and also the 1:1 rock salt ordering of the B-site cations. Diffuse reflectance spectra of Ba2Ln2/3TeO6 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 Ba2La2/3- xEu xTeO6 ( x = 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 Ωλ (λ = 2 and 4) were calculated, which indicate that Eu3+ ions occupy the symmetric octahedral B-site of the Ba2La2/3TeO6.