3 research outputs found
Structural Characterization of B‑Site Ordered Ba<sub>2</sub>Ln<sub>2/3</sub>TeO<sub>6</sub> (Ln = La, Pr, Nd, Sm, and Eu) Double Perovskites and Probing Its Luminescence as Eu<sup>3+</sup> Phosphor Hosts
Ba<sub>2</sub>Ln<sub>2/3</sub>TeO<sub>6</sub> (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 Ă… 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 LnO<sub>6</sub> and TeO<sub>6</sub> octahedra. In accordance with observed
number of bands and group theoretical predictions, the most likely
symmetry of all compounds in the Ba<sub>2</sub>Ln<sub>2/3</sub>TeO<sub>6</sub> system was found to be monoclinic with <i>P</i>2<sub>1</sub><i>/n</i> space group. Rietveld refinement
of the XRD patterns further confirmed the <i>P</i>2<sub>1</sub><i>/n</i> space group and also the 1:1 rock salt
ordering of the B-site cations. Diffuse reflectance spectra of Ba<sub>2</sub>Ln<sub>2/3</sub>TeO<sub>6</sub> 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 Ba<sub>2</sub>La<sub>2/3–<i>x</i></sub>Eu<sub><i>x</i></sub>TeO<sub>6</sub> (<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 Sr<sub>2</sub>Si<sub>5</sub>N<sub>8</sub>:Eu<sup>2+</sup>, which points toward the possible applicability of these new red
phosphors in solid-state lighting industry. Finally, Judd–Ofelt
intensity parameters Ω<sub>λ</sub> (λ = 2 and 4)
were calculated, which indicate that Eu<sup>3+</sup> ions occupy the
symmetric octahedral B-site of the Ba<sub>2</sub>La<sub>2/3</sub>TeO<sub>6</sub>