Site Preference-Driven Mn 4+ Stabilization in Double Perovskite Phosphor Regulating Quantum Efficiency from Zero to Champion.

The tetravalent-state stability of manganese is of primary importance for Mn 4+ luminescence. Double perovskite-structured A 2 B'B″O 6 :Mn 4+ has been recently prevalent, and the manganese ions are assumed to substitute for the B″(IV-VI)O 6 site to stabilize at the tetravalent charge state to generate far-red emissions. However, some Mn-doped A 2 B'B″O 6 -type materials show no or weak luminescence such as typical Ca 2 MgWO 6 :Mn. In this work, a cation-pair co-substitution strategy is proposed to replace 2Ca 2+ by Na + -La 3+ to form Ca 2-2 x Na x La x MgWO 6 :Mn. The significant structural distortion appears in the solid solution lattices with the contraction of [MgO 6 ] but enlargement of [WO 6 ] octahedron. We hypothesize that the site occupancy preference of Mn migrates from Mg 2+ to W 6+ sites. As a result, the effective Mn 4+ /Mn 2+ concentration enhances remarkably to regulate nonluminescence to highly efficient Mn 4+ -related far-red emission. The optimal CaNa 0.5 La 0.5 MgWO 6 :0.9%Mn 4+ shows an internal quantum efficiency of 94% and external quantum efficiency of 82%, reaching up to the top values in Mn 4+ -doped oxide phosphors. This work may provide a new perspective for the rational design of Mn 4+ -activated red phosphors, primarily considering the site occupancy modification and tetravalent-state stability of Mn.