engleski

XAFS investigations of Cr- and Eu-doped BaAl2O4

Barium aluminate, BaAl2O4, is a widely used material that has a high technological application in the field of electronics and optical communication. Namely, when BaAl2O4 is doped/co-doped with rare earth or transition metal cations, it exhibits good luminescence properties. Europium doped BaAl2O4 is already used as a luminescent material with emission in the red spectral range. Optical properties of this material have been widely investigated, but the crystal structure of Eu-doped BaAl2O4 has not been examined. A mechanism of Eu-doping into the BaAl2O4 structure was only predicted on the basis of optical data and DFT calculations. However the proposed crystal structure has not yet been proven by structure sensitive techniques. Chromium is another, low cost activator and Cr3+-doped systems are already used in modern technologies, for example for the production of solid-state lasers. The literature data on optical properties of Cr-doped BaAl2O4 are very rare, while structural data are completely missing. In this contex, we have conducted X-ray absorption measurements (XANES and EXAFS) of the Eu- and Cr-doped BaAl2O4 samples. Powder samples of undoped BaAl2O4 and BaAl2O4 doped with 4, 6 and 8 at.% Cr (in relation to Al) or Eu (in relation to Ba) were prepared by a hydrothermal method, additionally annealed at 1100 ºC for 4h. XRD showed that Cr- and Eu-doped samples possess a BaAl2O4-type structure with unit-cell parameters slightly different than that of pure BaAl2O4. Besides, Cr-doped samples contained a small amount of the impurity, namely BaCrO4. Cr K-edge fluorescence XANES spectrum from 4%Cr-doped BaAl2O4 sample suggests that Cr incorporated in the doped sample seems to be in a 6+ state (about 95% of the total Cr present in the sample) and in 3+ state (about 5% of the total Cr). EXAFS data for all the investigated Cr-doped samples gave Cr-O distances of about 1.66(1) Å, coordination numbers of ~4 and sigma2 of 0.0028 Å2. XANES/EXAFS experiments on Cr-doped samples suggest the inclusion of a small amount of chromium into the BaAl2O4 structure (as Cr3+) on tetrahedral Al-sites, and the formation of an impurity phase BaCrO4 (containing Cr6+). As indicted by the measured XANES spectra of the Eu-doped samples, only Eu3+ is present in the samples. In contrast to the Cr-doped materials, it can be expected that the Eu-ion is too large to occupy Al-sites in the host lattice. This is also reflected by the detailed analysis of the EXAFS data recorded from the samples, that suggest a mean Eu-O distance of about 2.24 Å, and substantially large coordination numbers of about 7-9. So it seems to be likely that on doping Eu fills the Ba-sites of the host BaAl2O4 lattice.

Barium aluminate ; Cr doping ; Eu doping ; X-ray absorption spectroscopy

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