Interdiffusion of solid iron and nickel at high pressure

Interdiffusion rates in solid (fcc) iron-nickel alloys have been measured at pressures between 1 and 23 GPa and temperatures between 1423 and 1973 K. The experiments were performed using piston cylinder and multianvil devices, with diffusion couples comprising pure iron and nickel rods placed end to end. Concentration profiles in quenched samples were measured by electron microprobe, and interdiffusion coefficients were determined using the Boltzmann-Matano method. Over the entire range of pressure and temperature studied, interdiffusion coefficients in the iron-rich alloys are described well by a simple exponential function of the homologous temperature, D = Doexp(− 20.4Tm / T), where Tm is the melting temperature of the alloy at the pressure of interest and D0 is a constant equal to 2.7 × 10− 4 m2/s. These data are consistent, within an order of magnitude, with data for other close-packed (fcc and hcp) metals at the same homologous temperatures. At the conditions of Earth's inner core, diffusion coefficients are predicted to be too small for bulk diffusive exchange between the inner and outer core to be significant. It is thus reasonable to model the chemical evolution of the outer core under the assumption that crystallization of the inner core is a perfect fractional process.