Site Dependence of Tb³⁺ Luminescence in Double Perovskite-Type Alkaline Earth Lanthanum Tantalates

Tb³⁺ doped alkaline earth lanthanum tantalates with the B-site-ordered double perovskite-type structure (AA′)[BB′]O₆ (namely, (CaLa₁–ₓTbₓ)[CaTa]O₆ (CLTO:Tb³⁺) and Ba₂[La₁–ₓTbₓTa]O₆ (BLTO:Tb³⁺) with x = 0.01, 0.10) were prepared, and their Tb³⁺ photoluminescence (PL) properties were examined and explained based on the occupied crystal sites (A or B sites). The locations of the Tb³⁺ activators were elucidated by analyzing powder X-ray diffraction patterns and X-ray absorption near edge structure at the Tb LIII edge. Tb³⁺ ions are located at A sites in CLTO:Tb³⁺ and at B sites in BLTO:Tb³⁺ as indicated in the nominal compositions. Both CLTO:Tb³⁺ and BLTO:Tb³⁺ exhibited intense PL excitation (PLE) peaks in the UV region due to the 4f–5d excitation for Tb³⁺. The PLE peak of Tb³⁺ in BLTO:Tb³⁺ showed a marked red-shift compared to that in CLTO:Tb³⁺ primarily because of the larger crystal fields at the B sites than at the A sites. The main PL peaks associated with the 4f–4f transitions from ⁵D₄ to ⁷F₅ states are also influenced by the crystal field, and the splitting of the PL peaks for Tb³⁺ at the B sites in BLTO:Tb³⁺ is more pronounced than that at the A sites in CLTO:Tb³⁺. In addition, owing to a downshift of the Tb 5d orbitals, the PL in the blue region derived from ⁵D₃–⁷FJ transitions is quenched for Tb³⁺ in BLTO:Tb³⁺.