Site Preference-Driven Mn⁴⁺ Stabilization in Double Perovskite Phosphor Regulating Quantum Efficiency from Zero to...

The tetravalent-state stability of manganese is of primary importance for Mn⁴⁺ luminescence. Double perovskite-structured A₂B'B″O₆:Mn⁴⁺ has been recently prevalent, and the manganese ions are assumed to substitute for the B″(IV-VI)O₆ site to stabilize at the tetravalent charge state to generate far-red emissions. However, some Mn-doped A₂B'B″O₆-type materials show no or weak luminescence such as typical Ca₂MgWO₆:Mn. In this work, a cation-pair co-substitution strategy is proposed to replace 2Ca²⁺ by Na⁺-La³⁺ to form Ca_2-2NaLaMgWO₆:Mn. The significant structural distortion appears in the solid solution lattices with the contraction of [MgO₆] but enlargement of [WO₆] octahedron. We hypothesize that the site occupancy preference of Mn migrates from Mg²⁺ to W⁶⁺ sites. As a result, the effective Mn⁴⁺/Mn²⁺ concentration enhances remarkably to regulate nonluminescence to highly efficient Mn⁴⁺-related far-red emission. The optimal CaNa_0.5La_0.5MgWO₆:0.9%Mn⁴⁺ shows an internal quantum efficiency of 94% and external quantum efficiency of 82%, reaching up to the top values in Mn⁴⁺-doped oxide phosphors. This work may provide a new perspective for the rational design of Mn⁴⁺-activated red phosphors, primarily considering the site occupancy modification and tetravalent-state stability of Mn.