HYDRATION REACTIONS OF NONSTOICHIOMETRIC BARIUM ORTHOALUMINATES

The hydration reactions of the nonstoichiometric barium orthoaluminate s having the chemical formula Ba1-xAl2O4-x (0<x<0.56) are studied. On exposure to CO2-free humid air, this compound reacted to form a new hy dration product, barium alumino hydroxy hydrate, Ba1-xAl2(OH)(8-2x). y H(2)O, wherein Al3+ is in octahedral coordination, as indicated by Al- 27 magic angle spinning nuclear magnetic resonance (MAS NMR). The form ation of hydrated product is faster for the nonstoichiometric composit ions (x > 0). The collapse of the nepheline-type structure of barium o rthoaluminate is explained in terms of the relative migration of Ba2ions from the structural channels. Increases in the reaction rate for the nonstoichiometric samples are attributed to faster diffusion of th e hydroxyl ions in the lattice, assisted by the structural vacancies. The product is highly sensitive to CO,; on exposure this compound give s rise to barium aluminium carbonato-oxyhydroxide and eventually segre gates to BaCO3 and aluminium hydroxide. On heating the hydrated produc t to 600 degrees C, the compound decomposed to Ba1-xAl2O4-x, indicatin g the reversibility of the reaction. The reactions involved, microstru ctural changes, and the possible mechanism of formation of hydrated pr oducts are discussed in detail. (C) 1998 Elsevier Science Ltd.