We developed a persistent phosphor of Y₃Al₂Ga₃O₁₂ doped with Nd³⁺, Ce³⁺, Cr³⁺ ions (YAGG:Nd-Ce-Cr) exhibiting long (>10 h) persistent luminescence at multi-wavelengths of around 880, 1064, and 1335 nm due to f-f transitions of Nd³⁺ and at 505 nm due to Ce³⁺:5d₁→4f transition. The intense near-infrared (NIR) persistent luminescence bands from Nd³⁺ match well with the first (650–950 nm) and second (1000–1400 nm) bio-imaging windows. The NIR persistent radiance of the YAGG:Nd-Ce-Cr phosphor (0.33 × 10⁻¹ mW/Sr/m²) at 60 min after ceasing blue light illumination was over 2 times higher than that of the widely used ZnGa₂O₄:Cr³⁺ red persistent phosphor (0.15 × 10⁻¹ mW/Sr/m²).

Narrower band red and green emission in phosphor‐converted white light‐emitting diodes (wLEDs) can improve the efficacy and color gamut in lighting and display applications. A promising luminescent ion is Mn²⁺ that can have both narrowband green (tetrahedral coordination) and red (octahedral coordination) emission. Unlike in earlier lighting applications of Mn²⁺ phosphors, temperature quenching is important in wLEDs. Insight into the thermal quenching behavior of Mn²⁺ luminescence is lacking. Here systematic research is reported for a variety of Mn²⁺‐doped phosphors; a huge variation in the luminescence quenching temperature T₅₀, ranging from 50 K for Mn²⁺ in ZnTe to 1200 K in MgAl₂O₄, is revealed. The value T₅₀ shows a positive correlation with the bandgap of the host, but no correlation with the full width half maximum (FWHM) of the emission band, indicating that thermally activated photoionization, not thermal crossover, is the operative quenching mechanism. This is confirmed by thermally stimulated luminescence (TSL) measurements that show a rise in TSL signal following photoexcitation at temperatures around T₅₀ providing evidence that quenching is correlated with generation of free charge carriers. Based on these findings, as a design rule is obtained that for temperature‐stable Mn²⁺ luminescence in (high power) LEDs a wide‐bandgap host material is required.