Site-Dependent Eu3+ Photoluminescence in Double Perovskite-Type Alkaline Earth Lanthanum Tantalates

Highlights: • Phase-pure Eu³⁺ doped Ca₂LaTaO₆ (CLTO) and Ba₂LaTaO₆ (BLTO) were prepared. • Eu³⁺ were confirmed to be at A sites in CLTO and B sites in BLTO by XANES analysis. • Eu³⁺ site-dependent PL spectra were obtained using the Eu³⁺-site-confirmed samples. • Site-dependent PL and the lifetime were examined systematically by CT excitation. • High external efficiency of 28% was found in the Eu³⁺ doped BLTO double perovskite. Site-dependent Eu³⁺ photoluminescence (PL) was examined utilizing the structural difference of two Eu³⁺ doped alkaline earth lanthanum tantalates with the B-site-ordered double perovskite-type structure (AA')[BB']O₆, viz. (CaLa_1−xEu_x)[CaTa]O₆ (CLTO:Eu³⁺) and (Ba₂)[La_1−xEu_xTa]O₆ (BLTO:Eu³⁺) (x = 0.01, 0.10). The locations of the Eu³⁺ ions were experimentally determined by X-ray absorption near edge structure of the Eu L_III edge and confirmed to be A sites in CLTO:Eu³⁺ and B sites in BLTO:Eu³⁺ as expected from the nominal compositions. Intense charge transfer (CT) excitation bands were observed in the UV region in the PL excitation spectra of both materials. Because of the shorter bond lengths and larger electronic repulsion at B sites than A sites, the CT excitation band of Eu³⁺ at B sites in BLTO:Eu³⁺ was blue-shifted compared to that of Eu³⁺ at A sites in CLTO:Eu³⁺. By UV excitation in the CT bands, both materials showed red luminescence. However, due to the difference in the site symmetry of A and B sites, the Eu³⁺ emission was mainly derived from the ⁵D₀ – ⁷F₂ transitions in CLTO:Eu³⁺ and from the ⁵D₀ – ⁷F₁ transitions in BLTO:Eu³⁺ in relation with Eu³⁺ at A and B sites, respectively. The lifetimes of the emission were also dependent on the Eu³⁺ sites resulting in the longer lifetimes of Eu³⁺ ions at B sites than those at A sites.