Boosting Photoluminescence of Rare-Earth-Based Double Perovskites by Isoelectronic Doping of ns² Metal Ions

Doping of ns² metal ions as an energy transfer (ET) bridge can significantly elevate the photoluminescence properties. Nonetheless, the fundamental influence of ns² metal ions on the local lattice structures remains unclear, hindering the advancement of functional materials. Herein, Sb³⁺ doped rare earth double perovskites is employed as a typical case to demonstrate this issue. It is found that the isoelectronic doping of Sb³⁺ ions not only enhances the ET efficiency but also changes their localized electronic and lattice structures. Both density functional theory (DFT) and Judd-Ofelt (J-O) theory calculations provide unambiguous evidence that the isoelectronic doping of Sb³⁺ ions enables a more localized charge density in the [LnCl₆]^3- (Ln: Lanthanide) octahedron and reduces the symmetry of the environment around the Ln³⁺, facilitating the radiative transition rates of Ln³⁺ while enhancing their ET efficiency. Compared with Cs₂NaScCl₆:Ln³⁺, the ET efficiency of Cs₂NaScCl₆:Sb³⁺/Ln³⁺ is enhanced by 1.5-fold, reaching up to 98.3%. To the best of available knowledge, this work is the first to unravel the intrinsic mechanism of enhanced ET process enabled by isoelectronic doping via DFT and J-O theory. This research sheds light on understanding the mechanism of photophysics and rational design of the functional perovskite materials.