Resonance Emission Enhancement (REE) for Narrow Band Red-Emitting A2GeF6:Mn4+ (A = Na, K, Rb, Cs) Phosphors Synthesized via a Precipitation–Cation Exchange Route
Narrow band red-emitting A2GeF6:Mn4+ (A = Na, K, Rb, Cs) phosphors were prepared through a two-step precipitation–cation exchange route using a K2MnF6 precursor as the Mn4+ source. The phase purity, morphology, and constituent were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectric spectroscopy (XPS), and electron paramagnetic resonance (EPR) examination. Optical properties were investigated by photoluminescence spectra (PL and PLE) and high-resolution PL. A temperature-dependent PL examination was performed to investigate the electron–phonon coupling emission mechanism of Mn4+ in these alkali fluorogermanates. The PL data show that both ordered distribution and appropriate distance between Mn4+ ions are propitious for enhancement of the emission intensity. A resonance emission enhancement (REE) mechanism has been proposed to explain the intensity increment among these products. These phosphors present bright red emission under blue light (467 nm) illumination, among which Cs2GeF6:0.03Mn4+ exhibits the most excellent optical properties with a quantum yield (QY) of 93%. A WLED (white light-emitting diode) fabricated with blend of commercial YAG:Ce3+ and this phosphor emits intense warm white light with low color temperature (CCT = 3385 K) and high color rendering index (Ra = 90.5), implying its potential application as red phosphor in WLEDs.
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