Diffusion of Iron and Nickel in Magnesium Oxide Single Crystals

Abstract

Diffusion kinetics and mechanisms were studied in the FexO‐MgO (vacuum), NiO‐MgO (vacuum and air), and Fe203‐MgO (air) systems. In the FexO‐MgO system, Fe entered MgO by a redox reaction; the diffusivity and activation energy depended on concentration. In the NiO‐MgO system in air the diffusivity depended on concentration and the activation energy did not; in vacuum both the diffusivity and activation energy were concentration‐independent. In the Fe2O3‐MgO system in the MgO phase the activation energy and diffusivity did not depend on concentration. Because of impurities, the diffusion results were for the extrinsic region. Formation of trivalent ions and consequent chemically created vacancies in the FexO‐MgO and NiO‐MgO (air) systems resulted in the concentration dependence of diffusivity. Concentration dependence of activation energy in the FexO‐MgO system is associated with structural changes due to a change with concentration of the Fe3+ octahedraI/Fe3+ tetrahedral ratio. In the Fe2O3‐MgO system structural changes do not occur during diffusion because this ratio remains constant.

Calculation of absolute diffusion rates in oxides

“…The review by Wuensch (1983) quotes activation energies of around 1.8 eV for Fe (arguably Fe2+ not Fe3*) in MgO. The value for the activation energy found from tracer diffusion experiments by Blank and Pask (1969) is 3.2 eV. The agreement between this value and our calculated energy difference of 3.18 eV between the equilibrium and midpoint configurations is almost certainly coincidental.…”

“…Experimentally, there are several sets of diffusion data for Fe in MgO (see Blank and Pask 1969). However, these present problems in estimating jump frequencies.…”

Novel preparation and properties of magnesioferrite nanoparticles

“…Blank and Pask [51] reported that the appearance of any Fe 3+ ions in MgO by diffusion would contribute to the chemically created vacancies depending upon the ionic radii of ferric and magnesium species are 0.063 and 0.065 nm, respectively [52]. In other words, 3Mg 2+ could be replaced by 2Fe 3+ and a vacancy because of electro-neutrality restrictions.…”

Incorporation of Mg and Ca into Nanostructured Fe2O3 Improves Fe Solubility in Dilute Acid and Sensory Characteristics in Foods

“…This expansion is probably caused by the incorporation of Mg 2+ in the tetrahedral lattice sites. The γ -Fe 2 O 3 structure allows ready incorporation of Mg 2+ into its lattice since both γ -Fe 2 O 3 and MgFe 2 O 4 exhibit the same inverse spinel (cubic) structure (Cornell and Schwertmann 2003) and Mg 2+ has a higher preference for octahedral positions than Fe 3+ (Blank and Pask 1969). The MgO cell constant was constant for the compound with a molar ratio of Mg : Fe ≥ 1.9, suggesting that MgO is the primary crystal structure.…”