Oxidation and Luminescence Quenching of Europium in BaMgAl₁₀O₁₇ Blue Phosphors

Abstract

The efficient blue luminescence of Eu 2+ doped BaMgAl 10 O 17 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 Eu 2+ 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 Eu 2+ survives degradation, implying additional mechanisms for the quenching of the luminescence. Defects created during the photogeneration may reduce the ability of the remaining Eu 2+ to receive or to radiatively release energy. A kinetic Monte Carlo simulation confirms that defects in the vicinity of a photogenerated Eu 3+ can act as killer centers for the remaining Eu 2+ 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.

X-ray absorption spectroscopy and Eu3+-emission characteristics in GaAs/SnO2 heterostructure

“…X-ray absorption near edge structure (XANES) is the features up to 50 eV after the absorption edge, being strongly sensitive to oxidation state and coordination chemistry of the absorber atom [12]. Amidani and coworkers [13] employed X-ray absorption spectroscopy (XAS) to understand the degradation process at atomic level in Eu 2+ -doped BaMgAl 10 O 17 . Data of high energy resolution fluorescence detected (HERFD)-XANES yielded stable structural properties of the host lattice upon irradiation, and rapid oxidation of Eu 2+ to Eu 3+ .…”

Suppression of Eu2+ Luminescence Loss

“…The valence state stability of Eu 2+ is the other factor to determine the luminescence performance of material because the possible intervalence CT process will severely deteriorate the luminescence efficiency of Eu 2+ . [ 51 ] Figure a gives the normalized Eu L 3 ‐edge X‐ray absorption near edge structure (XANES) spectra of Eu 2+ ‐doped SMPO. A predominant absorption at 6974 eV attributing to Eu 2+ 2p 3/2 →5d transition is clearly observed, [ 52 ] showing that the europium ions mostly exist in +2 state in the system (Figure S10, Supporting Information).…”

“…Only the intense Eu 2+ luminescence is observed, which confirms the absence of Eu 3+ in the system. The predominance of Eu 2+ is not common in the nominal Eu 2+ ‐doped oxide compounds, [ 26,54 ] and even in some commercial‐used phosphors, [ 9,51a,55 ] a certain amount of Eu 3+ is detectable. The closeness of Eu 2+ 4f ground state to the Fermi level of current system is perhaps the reason for this Eu 2+ predominance.…”

New reflections on hard X-ray photon-in/photon-out spectroscopy

“…A possible explanation invokes "killer" centers of a certain radius around oxidized Eu that prevent transfer of energy to Eu 2+ to let it luminesce. 68 Figure 7 X-ray induced oxidation of Eu 2+ observed in Lα1-detected Eu L3 edge HERFD-XANES on BAM:Eu. The inset shows the decay of Eu 2+ XANES peak (blue) and of the integrated luminescence (black) during irradiation together with the results of a Monte Carlo (MC) simulation (red).…”