Structural Confinement and Energetic Matching Synergistic Effect toward a High-Energy Transfer Efficiency and a Significant Red Emission Enhancement in a Eu 2+ ,Ln 3+ Co-doped Sr 9 LiMn(PO 4 ) 7 Whitlockite Phosphor.

Despite an encouraging progress, Mn 2+ -activated red phosphors suffer from an insufficient emission intensity and a bad color purity. Thus, it is necessary to find a new strategy to realize a bright red emission through highly efficient Mn 2+ sensitization. Herein, manipulating Eu 2+ -sensitized Sr 9 LiMn(PO 4 ) 7 (SLMP) composition by Ln 3+ heterovalent substitution is proved to be able to substantially gain a tremendous Mn 2+ emission enhancement and result in a dominant red Mn 2+ emission. It is found that the emission enhancement ratio is proportional to the order of lanthanide contraction. Notably, Tb 3+ doping realizes a 427-fold rise in the integrated emission intensity compared with the SLMP host, which is close to the theoretical maximum of 500. An underlying mechanism for Mn 2+ red emission enhancement is proposed, which is attributed to a high-energy transfer probability from Eu 2+ to Mn 2+ via Ln 3+ -induced further structural confinement plus an energetic match effect. Meanwhile, homovalent (Ca 2+ ) substitution could precisely tailor Mn 2+ emitting color from orange-red to deep red. A warm-white LED device with a low color temperature of 3394 K, a high color-rendering index of 90.2, and suitable CIE coordinates of (0.403, 0.373) is fabricated using optimized phosphor SLMP:Eu 2+ , Tb 3+ . These results might reveal a new strategy to develop new red-emitting phosphors with a bright and highly purified red Mn 2+ emission.