Garnet-type crystallites, their isomorphism and luminescence properties in glass ceramics
Glass ceramics based on Ce-doped garnets were fabricated by the controlled crystallization of CaO(MgO)–Y2O3–Al2O3–GeO2–SiO2–CeO2 glasses. The optimal conditions for transforming these glasses into ceramics by heat treatment were found by using differential scanning calorimetry: 1400 °C for MgO–Y2O3–Al2O3–GeO2–SiO2–CeO2 and 1500 °C for CaO–Y2O3–Al2O3–GeO2–SiO2–CeO2. The structure was characterized by X-ray diffraction (XRD) and energy-dispersive X-ray spectroscopy techniques, while the emission properties were studied by photoluminescence (PL) spectroscopy. The fine structure of XRD patterns evidences contributions from garnets of different compositions. Spatially-resolved PL spectroscopy using confocal microscopy shows that the garnet crystallites have a layered structure. The capability of tuning the PL peak position within the range from 530 to 580 nm by changing the excitation wavelength was demonstrated. This effect is caused by the localization of Ce3+ ions at the positions with different crystal fields, which varies due to a composition gradient in the crystallites. These glass ceramics have high potential for exploitation as phosphors in high-power white light sources.