Cation Loss of BaCa0.393Nb0.606O2.91 in Aqueous Media: Amorphization at Room Temperature

Abstract

The complex perovskite BaCa(1+x)/3Nb(2−x)/3O3−x/2 (here x=0.18), also referred to as BCN18, is a widely studied high-temperature proton conductor which has O-vacancies in the dry state accommodating highly mobile protonic defects 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 this layer ranges from tens of nm to several μm depending on exposure time. The amorphous layer is studied with TEM, XRD, XPS, SNMS, TRFA, and AFM. It is 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 amorphous phase of Nb, O, and some water. A prerequisite of the above scheme inferred from the kinetics of the process is the presence of planar defects or channels of amorphous character in the initial BCN18 structure that allow rapid diffusion of Ba, Ca, and H. The present model assumes a block arrangement for the initial BCN18 with perfect perovskite structure inside the blocks, separated by thin amorphous slabs.