Phase separation and crystallization processes in heat treatment of Mn4 +-doped 90GeO2–10BaO glass and fluorescence properties

•Surface layer of 90GeO2–10BaO–0.1MnO glass is crystallized at 700 °C.•90GeO2–10BaO–0.1MnO glass is homogeneously crystallized at 800 °C by phase separation.•Mn4 +:BaGe4O9 nanocrystal is formed in BaO-rich phase of the glass at 800 °C.•Mn4 + red fluorescence band appears at the peak wavelengths of 642, 653, 669, 683, and 693 nm.•Glass–ceramics heated at 800 °C for 102.0 s show the strongest Mn4 + red fluorescence.

To prepare transparent 90GeO2–10BaO–0.1MnO glass–ceramics for Mn4 + red fluorescence, a glass was prepared by a melt-quenching method, and then the glass was crystallized at 700 and 800 °C for various seconds. The crystallization processes were investigated by measuring and observing the glass–ceramics using an XRD analyzer, an IR absorption spectrophotometer, a field emission scanning electron microscope, and a fluorescence spectrometer. As a result, for the heat treatment at 700 °C, α-GeO2 and BaGe4O9 crystals doped with Mn4 + ions were formed with oxidation of Mn2 + ions to Mn4 + ones in the surface layer by heterogeneous nucleation. For the heat treatment at 800 °C, by homogeneous crystallization with nano-sized phase separation, α-GeO2 and Mn4 +:BaGe4O9 nanocrystals were formed in a GeO2-rich phase and an Mn4 + ions-containing BaO-rich phase, respectively. In this study, transparent 90GeO2–10BaO–0.1MnO glass–ceramics crystallized at 800 °C for 102.0 s by homogeneous crystallization with nano-sized phase separation, showed the strongest red fluorescence band due to the 2E(2G) → 4A2(4F) transition of Mn4 + ions in the BaGe4O9 nanocrystals.

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