Light-Induced Degradation in Red-Emitting ZnSiF6·6H2O:Mn4+ Hydrate Phosphor

Photo-induced degradation behaviors of the red-emitting ZnSiF6·6H2O:Mn4+ hydrate phosphor are studied using X-ray diffraction measurement, photoluminescence (PL) analysis, PL excitation spectroscopy, PL decay analysis, diffuse reflectance spectroscopy, and electron spin resonance (ESR) measurement. The ZnSiF6·6H2O:Mn4+ hydrate phosphor exhibits remarkable degradation in the PL intensity under Xe lamp exposure for a few minutes. Coherent laser beams also induce degradation and its degree is in the order of Ar+ (488 nm) > He−Cd (325 nm) > He−Ne laser (632.8 nm). The degradation mechanism is found to be due to change in the valence state of manganese ions from Mn4+ to Mn5+ by the photooxidation (Mn4+ → Mn5+) or disproportionation reaction (2Mn4+ → Mn3+ + Mn5+). This is supported by the facts that Mn4+ ion has larger absorbance at 488 nm than at 325 or 632.8 nm and that the Mn5+-related absorption structures appear in the diffuse reflectance spectra at wavelengths below ∼700 nm. The ESR measurement also confirms the decreased Mn4+ spin density in the sample exposed with Xe lamp. A comparative discussion is given on the PL properties of the two different hydrate phosphors, ZnSiF6·6H2O:Mn4+ and K2SnF6·H2O:Mn4+.