A far-red-emitting NaMgLaTeO6:Mn4+ phosphor with perovskite structure for indoor plant growth

In this article, we report a far-red-emitting phosphor NaMgLaTeO6:Mn4+ with the perovskite structure synthesized by a high-temperature solid-state reaction method. The XRD patterns for NaMgLaTeO6:Mn4+ and the refinement for the representative NaMgLaTeO6:0.006Mn4+ confirm the purity of the as-prepared samples. The emission spectrum of optimal NaMgLaTeO6:0.02Mn4+ covers a series of narrow lines in the range from 16667 to 12500 cm−1 (600–800 nm), peaking at 14225 cm−1 (703 nm) due to the Mn4+ spin- and parity-forbidden 2Eg→4A2g transition. The excitation spectrum monitored at 703 nm has a broad excitation band with the range from 40000 to 16667 cm−1 (250–600 nm) centered at 28571 (350 nm) and 21505 cm−1 (465 nm), which matches well with UV or blue chips, respectively. The band can be decomposed into four Gaussian bands centered at 31646 cm−1 (316 nm), 28169 cm−1 (355 nm), 24631 cm−1 (406 nm) and 20921 cm−1 (478 nm), corresponding to the O2--Mn4+ charge-transfer and the Mn4+ 4T1g←4A2g, 2T2g←4A2g and 4T2g←4A2g transitions, respectively. The optimal concentration x of Mn4+ in NaMgLaTeO6:xMn4+ was examined to be 0.02 with a corresponding quantum yield of 57.43% under 365 nm excitation, beyond which concentration quenching occurred. The energy transfer mechanism between Mn4+ ions was determined to be a dipole-dipole interaction by analyzing the relationship between concentration and emission intensity. The temperature-dependent luminescence intensity decreased to about 75% at 150 °C of its original value at room temperature, illustrating its thermal luminescence stability of this phosphor. Moreover, a crystal field analysis for the NaMgLaTeO6:0.02Mn4+ representative was conducted. By using the decay lifetimes and time-resolved emission spectra, we discriminated the different occupancies of Mn4+ not only at the Te6+ but also at the Mg2+ sites in the crystal structure. More importantly, the emission band matched well with the absorption band of phytochrome Pfr, which implies the phosphors can be potentially applied in light-emitting diodes (LEDs) for regulating the growth of plants.