Preparation of barium hexa-aluminate through a hydrothermal precipitation–calcination route and characterization of intermediate and final products

high surface area (f22 m2/g) micro-sized (0.3–2 A) barium hexa-aluminate powder was prepared by the calcination (at
1400 jC for 2 h) of hydrothermally prepared precursors of orthorhombic barium carbonate and boehmite. The precursors were
obtained by a hydrothermal treatment (at 180 jC, for 1 h) of aqueous solutions of barium and aluminum nitrates of appropriate
compositions in the presence of aqueous urea as the neutralizing precipitant. Results of XRD, TG-DTA and SEM measurements
suggest the most probable reaction sequence for the preparation of barium hexa-alumnate ceramic powder as: (i) the
hydrothermal precipitation of orthorhombic BaCO3 and boehmite (g-AlOOH) precursors from the aqueous Ba and Al nitrates in
the presence of urea, (ii) the formation of an interim mixture of g-Al2O3, BaCO3 and Ba–h1-alumina in the temperature range
of 800–1200 jC, and finally (iii) the formation of a ceramic powder corresponding to the composition BaO6Al2O3 having halumina
type structure at f1400 jC. The final material did not show presence of any other crystalline phases like alumina,
baria, barium mono-aluminate and tri-barium mono-aluminate phases.