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Dopant environment within Eu-doped BaAl2O4: a complementary XRPD and XAS study

Barium aluminate BaAl2O4 is a photoabsorptive material of a considerable interest because of its potential application in the field of electronics and optical data processing (Kim et al., 2000). The electronic properties, as well as the photoabsorption performance of this material, can be tuned by addition of varying amounts of dopants/defects into the host structure resulting with more promising or advanced applications (Zhang et al., 2007). When doped/co-doped with transition metal cations or rare earth cations, it exhibits good luminescence performance. The europium doped BaAl2O4 is already used as a luminescent material with an emission in the red spectral range. Optical properties of this material have been widely investigated, but the crystal structure has not been examined. A mechanism of Eu incorporation into the BaAl2O4 host was only predicted on the basis of optical data (Peng & Hong, 2007). Barium aluminate exists in two hexagonal phases with a reversible phase transition at 123 °C (Huang et al., 1994). At RT a ferroelectric phase in P63 space group is present, while at high temperature it holds a P6322 space group assigned to the paraelectric phase. Structural studies of nanocrystalline powder sample of BaAl2O4 doped with 4.9 at% Eu3+ have been performed in order to examine the dependence of dopant incorporation mechanism on its oxidation state and doping level. Eu3+-doped BaAl2O4 phosphor sample was prepared by the hydrothermal route. Combined X-ray powder diffraction (XRPD), 151Eu Mössbauer measurements and synchrotron based X-ray absorption spectroscopy (XAS) were applied in order to examine the location and oxidation state of the europium dopant within the BaAl2O4 host matrix. Obtained XANES data at Eu L3-edge for the Eu- doped sample showed the presence of europium in 3+ oxidation state solely. An occupation of barium sites in the host BaAl2O4 lattice was clearly evidenced within a detailed analysis of the EXAFS data at the Eu L3-edge for the Eu-doped sample. The results suggested a mean Eu-O distance of about 2.40 Å and the coordination number of about 9, both indicating on Eu3+ substitution for Ba2+. Results of the Rietveld structure refinement analysis have indicated that Eu3+ dopant ions tend to occupy IX-coordinated Ba2 positions in hexagonal channels formed by corner-linked AlO4 tetrahedra of the BaAl2O4 host with a charge compensation by oxygen interstitials. References: Huang, S.-Y., Von der Muell, R., Ravez, J., Chaminade, J. P., Hagenmuller P. & Couzi M. J. (1994). Solid State Chem. 109, 97-105. Kim, C., Kwon, I., Park, C., Hwang, Y., Bae, H., Yu, B., Pyun, C. & Hong. G. (2000). J. Alloys Compd. 311, 33-39. Peng, M. & Hong, G. (2007). J. Lumin. 127, 735- 740. Zhang, L., Wang, L. & Zhu, Y. (2007). Adv. Funct. Mater. 17, 3781-3790.

X-ray powder diffraction, Rietveld method, EXAFS, XANES

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