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Structural Investigations of Cr-Doped Barium Aluminate

Barium aluminate, BaAl2O4, is a widely used material that has a high technological application in the field of electronics and optical communications. When doped/co-doped with transition metal cations or rare earth cations, it exhibits good luminescence properties [1]. Barium aluminate exists in two hexagonal phases with a reversible phase transition at 123 °C. At RT a ferroelectric phase in P63 space group is present, while at high temperature a paraelectric phase in P6322 space group is observed [2]. Powder BaAl2O4 samples doped with 0 and 1.76 at.% Cr in relation to Al were prepared by a hydrothermal method. Prepared samples were characterized by X-ray diffraction and synchrotron based X-ray absorption spectroscopy at the Cr K- and Ba L3-edge. XRD patterns showed that samples were nanocrystalline with a hexagonal crystal structure, space group P63. The doped sample contained a small amount of an impurity phase BaCrO4. Obtained Cr K-edge X-ray absorption near edge structure for the doped sample showed the presence of chromium in 6+ and 3+ oxidation states. Evidently, the Cr6+ ion was present in BaCrO4, while Cr3+ likely participated in the formation of the BaAl2O4:Cr phase. Extended X-ray absorption fine structure indicated on an unusual tetrahedral coordination of Cr3+ ions within the BaAl2O4 host phase. The structure of samples was refined by the Rietveld method [3]. Rietveld structure refinement showed that in doping Cr3+ ions substituted for Al3+ ions on Al1 tetrahedral sites of the BaAl2O4 crystal lattice. Lattice strain increased with Cr doping, while crystallite size simultaneously decreased. [1] Fukuda K., Iwata T., Orito T. J. Solid State Chem., 2005, 178, 3662. [2] Huang S.- Y., Von der Muell R., Ravez J., Chaminade J. P., Hagenmuller P., Couzi M. J. Solid State Chem., 1994, 109, 97. [3] Rietveld H. M. J. Appl. Crystallogr., 1969, 2, 65.

X-ray diffraction; X-ray absorption spectroscopy; Rietveld structure refinement

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