Competing Energy Transfer-Modulated Dual Emission in Mn²⁺-Doped Cs₂NaTbCl₆ Rare-Earth Double Perovskites

A₂B^IB^IIIX₆ double perovskites are promising materials due to their outstanding photoelectronic properties and excellent stability in the environment. Herein, we synthesized Mn²⁺:Cs₂NaTbCl₆ with dual emission through a solvothermal method for the first time. Mn²⁺:Cs₂NaTbCl₆ double perovskites exhibit excellent environmental stability and high photoluminescence quantum yields (PLQYs). The Cs₂NaTbCl₆ was successfully doped with Mn²⁺ in two modes: at Mn-feeding concentrations below 1%, Mn²⁺ first tend to insert into the interstitial void, but if the Mn-feeding concentration exceeds 1%, Mn²⁺ will further substitute Na⁺ site of the Cs₂NaTbCl₆ lattice and thus both two doping modes coexist. After Mn²⁺ doping, efficient energy transfer from the ⁵D₄ level of Tb³⁺ ions to the ⁴T₁ level of Mn²⁺ ions occurs, resulting in tunable dual emission from the Tb³⁺⁵D₄ → ⁷F_J=6,5,4,3 transition and Mn²⁺⁴T₁ → ⁶A₁ transition. Further, LED based on the Mn²⁺:Cs₂NaTbCl₆ double perovskites exhibits excellent performance and stability. This work demonstrates a strategy to achieve novel lanthanide-based double perovskites with potential applications in photonics.