Advanced probing of Eu2+/Eu3+ photoemitter sites in BaAl2O4:Eu scintillators by synchrotron radiation X-ray excited optical luminescence probe

Abstract

The synchrotron radiation hard X-ray photons exhibit several advantages, such as monochromaticity, energy tunability and a desired penetration depth. This allows to precisely probe the chemical environment of the Eu²⁺/Eu³⁺ dopants' sites in a host lattice by X-ray absorption fine structure (XAFS) and X-ray excited optical luminescence (XEOL). Thus, tuning the photon energy of monochromatic X-rays around the L-edges of Europium and combining XAFS with the XEOL data provided an ideal probe for determining the oxidation state, local coordination environment and optical behavior of the Eu²⁺/Eu³⁺ sites. XEOL spectra of the X-ray induced luminescent samples S1 (BaAl₂O₄:Eu-BaCO₃) and S2 (BaAl₂O₄:Eu-BaCO₃/Al(OH)₃), measured under excitation of tunable Eu L_3,2,1-edges X-ray energies (7150, 7710, and 8150 eV) manifested that Eu²⁺ occupies more than one Ba sites in BaAl₂O₄ lattice, with a probable contribution from the BaCO₃. The Eu and Ba local site-specific L₃-edges X-ray absorption near edge structure (XANES) spectra, Continuous Cauchy wavelet transform (CCWT) and quantitative analyses of the extended X-ray absorption fine structure (EXAFS) data, demonstrated a mixed occupancy of Eu/Ba at the cation sites in BaAl₂O₄ lattice, dominantly for the S1 phosphor. However minor contribution from Eu site in the BaCO₃ cannot be neglected, as evidenced form the quantitative EXAFS fit of S1 phosphor. Quantitative phase analysis accomplished by Rietveld refinements of the experimental XRD patterns indicated the considerable enhancement in yield of BaAl₂O₄:Eu (S1) sample, irradiated with IR light, when compared to the S2. The emitting center's site specific XEOL spectroscopy in combination with XAFS validated the existence of europium predominantly in +3 oxidation state with minute Eu²⁺ impurity occupying multiple sites in the host lattice for the S1 and S2 phosphors.