Chemical diffusion in dense polycrystalline oxide superconductors was studied gravimetrically at temperatures of between 550 and 850C, under an O partial pressure of between 1 and 0.01atm. It was found that the chemical diffusion coefficient was markedly dependent upon the O content. Thus, its value increased sharply with decreasing O deficiency but did not change appreciably at the structural transition between the orthorhombic and tetragonal phases. This compositional dependence of the diffusivity was not expected for a simple vacancy diffusion mechanism. It was instead attributed to a composition-sensitive activation energy for the 2-dimensional motion of O within the unusual crystal structure.

K.Kishio, K.Suzuki, T.Hasegawa, T.Yamamoto, K.Kitazawa, K.Fueki: Journal of Solid State Chemistry, 1989, 82[2], 192-202