Diffusion in Oxides

Abstract

Non-silicate oxide minerals are minor but important constituents of many igneous and metamorphic rocks, a major component of Earth’s lower mantle, and are well represented in planetary and meteoritic materials. Oxide minerals also have important roles in technology, for example as semiconductors, thermal and electrical insulators, fuel cell components, substrates for thin films, photovoltaic materials, and as products of metal oxidation. Because of their technological applications, and their fundamental interest to geosciences, materials science, physics and chemistry, the diffusion properties of many oxide minerals have been studied intensively, using a wide range of experimental, analytical and computational approaches. In many cases, particular attention has been devoted to deciphering the atomic-level mechanisms involved in diffusion. With the possible exceptions of metals and halides, oxides have probably been studied in more detail with regard to their point defect and diffusion properties than any other group of minerals. The oxide minerals considered in this chapter are relatively simple in terms of their structure and chemistry, but nonetheless exhibit quite complicated diffusion behavior in many cases. Due to the simplicity of the minerals, and to the amount and quality of data available, the origin of many of these complicated behaviors is fairly well understood. In magnetite, for example, cation diffusion rates have a complex dependence on oxygen fugacity. This dependence is due to internal redox reactions, and to a transition from an interstitial diffusion mechanism at low oxygen fugacities to a vacancy mechanism under more oxidizing conditions. Oxygen and titanium diffusion rates in rutile also vary strongly with f O2, due to internal reduction of titanium and the associated production of oxygen vacancies and titanium interstitials. Some cations in rutile exhibit strong diffusional anisotropy, which is thought to result from rapid diffusion along interstitial “channels” that extend along the c direction in …