10 - Diffusion in Some Perovskites: HTSC Cuprates and a Piezoelectric Ceramic

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A large body of data now exists in the literature on diffusion of cations and anions in a variety of high-temperature superconducting cuprates and some perovskites, which are reviewed and critically discussed in this chapter. Diffusion data are available on the YBa2Cu3O7-x superconductor of the constituent cations, Cu, Ba, and Y, and also of the impurity Ni, Co, Ag, and Zn cations. Self-diffusion of Cu takes place largely on the Cu sites through thermal vacancies with an activation energy of 255 kJ/mol. Self-diffusion of Ba and Y atoms also takes place on their respective sites and is characterized by much larger activation energies, in the 900 to 1000 kJ/mol range, because they are deeply embedded in the lattice. The large values of activation energies for Ba and Y reflect the difficulty of the diffusion process due to long atomic-jump distances and the lack of vacancies in which jumps can occur. The Ni, Co, Ag, and Zn impurity cations have diffusion parameters similar to the Cu self-diffusion in YBa2Cu3O7-x because they all replace Cu. Some effect of ion charge is seen in the case of heterovalent cations. Extensive diffusion data on the anion species (O) also exist in about a dozen high-temperature superconducting cuprates, which are compiled and compared. The oxygen diffusion data show scatter of many orders of magnitudes. This is attributed to several factors, notably, the deviation from stoichiometry and the atomic arrangements in the various Cu–O planes, which differ from material to material. These factors, in fact, determine the upper and lower limits of the diffusion data. A comparison is also made between the diffusion kinetics of the cation and anion species.