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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.
