Oxidation and Luminescence Quenching of Europium in BaMgAI10O17 Blue Phosphors

The efficient blue luminescence of Eu2+ doped BaMgAl10O17 is well-known to be severely degraded by prolonged irradiation with vacuum-ultraviolet light. The degradation process at the atomic level is however not fully understood. In this work we employed X-rays as an equivalent but accelerated cause of degradation, as an excitation source of luminescence and as an element-selective probe of both dopants and host-lattice chemical species. The X-ray absorption near edge structure (XANES) recorded in high energy resolution mode reveals that the structural properties of the host lattice are preserved during irradiation, while Eu2+ is rapidly oxidized. The correlation between Eu oxidation as derived from XANES and the decrease of blue luminescence is however not linear and a significant fraction of Eu2+ survives degradation, implying additional mechanisms for the quenching of the luminescence. Defects created during the photogeneration may reduce the ability of the remaining Eu2+ to receive or to radiatively release energy. A kinetic Monte Carlo simulation confirms that defects in the vicinity of a photogenerated Eu3+ can act as killer centers for the remaining Eu2+ and explain the observed accelerated quenching of the blue luminescence. The present approach, which takes full advantage of the different interactions of X-ray radiation with impurity doped luminescent materials, can easily be transferred to reveal degradation pathways in other phosphors.