Optical spectroscopy and degradation behavior of ZnGeF6·6H2O:Mn4+ red-emitting phosphor

We report on a method of synthesizing Mn4+-activated ZnGeF6·6H2O hexahydrate red phosphor by the chemical reaction and its photo-induced degradation behavior. The structural and optical properties of ZnGeF6·6H2O:Mn4+ are investigated using X-ray diffraction measurement, photoluminescence (PL) analysis, PL excitation (PLE) spectroscopy, PL decay measurement, diffuse reflectance measurement, and electron spin resonance (ESR) spectroscopy. The present hexahydrate phosphor exhibits the sharp red emission lines typically observed in various Mn4+-activated phosphors. However, the phosphor shows remarkable PL intensity degradation under Xe lamp exposure for a few minutes or coherent laser beam irradiation at different wavelengths. Its degree is in the order of Ar+ (488 nm)>He-Cd (325 nm)>He-Ne laser (632.8 nm). The photo-induced degradation mechanism is explained by a change in the valence state of manganese ions from Mn4+ to Mn5+ by the photooxidation (Mn4+→Mn5+) or disproportionation reaction (2Mn4+→Mn3++Mn5+). The ESR measurement confirms the decreased Mn4+ spin density in the sample exposed with Xe lamp after Xe lamp exposure for 5 min. The photo-induced degradation is concluded to universally occur in AIIBIVF6·6H2O:Mn4+ hexahydrate phosphors.