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Abstract

Combining laboratory X-ray powder diffraction with in-situ high-temperature synchrotron experiments and differential scanning calorimetry, it has been shown that Ba21Al40, Ba3Al5, Ba7Al10 and Ba4Al5 decompose peritectically at 914, 826, 756, and 732 degrees C, respectively. In addition, a new binary compound with the composition Ba4Al7+x (x = 0.17) and the formation temperature of 841 degrees C was found. The initial structural model (space group P6(3)/mmc, a = 6.0807(1), c = 39.2828(8)angstrom) with four Ba and five Al crystallographic positions was developed. It is based on the intergrowth concept involving the neighboring Ba21Al40 and Ba3Al5 phases and the derived atomic arrangement is subsequently refined using X-ray diffraction data. The crystal structures of all phases in the Ba-Al system, except BaAl4, exhibit Kagome nets of aluminum atoms resembling those observed for the B atoms in the Laves phases AB(2). In the crystal structure of Ba4Al7+x, single Kagome layers alternate with double slabs (MgZn2 motif) along [001] and are separated by Ba cations. Inter-growth features of Ba4Al7+x are discussed together with the neighboring Ba-Al compounds and Sr5Al9.