Cation loss of BaCa0.393Nb0.606O2.91 in aqueous media: Amorphization at room temperature

The complex perovskite BaCa(1+x)/3Nb(2-x)/3O3-x/2 (here x = 0.18), also ref erred to as BCN18, is a widely studied high-temperature proton conductor wh ich has O-vacancies in the dry state accommodating highly mobile protonic d efects in the wet state. Experimental evidence is given that BCN18 exposed to a very dilute acid (here HCl), or even hot water, undergoes a remarkable reaction to an amorphous state within a surface layer. The thickness of th is layer ranges from tens of nm to several pm depending on exposure time. T he amorphous layer is studied with TEM, XRD, XPS, SNMS, TRFA, and AFM. It i s concluded that Ba and Ca are almost completely leached out of the surface layer. Protons penetrate into the solid, forming an intermediate, unstable solid acid, rich in Nb and O. The latter eventually collapses into an amor phous phase of Nb, O, and some mater. A prerequisite of the above scheme in ferred from the kinetics of the process is the presence of planar defects o r channels of amorphous character in the initial BCN18 structure that allow rapid diffusion of Ba, Ca, and H. The present model assumes a block arrang ement for the initial BCN18 with perfect perovskite structure inside the bl ocks, separated by thin amorphous slabs. (C) 2000 Academic Press.