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

Abstract

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+.