Invariant electrical resistivity of Co along the melting boundary

The Earth's core is comprised mainly of Fe and Ni with some light alloying element(s) and the electrical resistivity behavior of these elements is an important property for characterizing geodynamo action, determining energy sources, and for understanding core thermal evolution. Knowledge of the electrical resistivity of solid and liquid transition metals with electronic structures similar to Fe reinforces our understanding of core properties. The electrical resistivity of high purity Co has been measured at pressures up to 5 GPa in a large volume press and at temperatures up to 100 K above the melting temperature. The results demonstrate that resistivity of Co is invariant along the melting boundary. This is interpreted in terms of the antagonistic effects of P-induced reduction in the amplitude of lattice vibrations tending to decrease resistivity, and the P-induced shift of the Fermi level closer to the d-resonance which tends to increase resistivity. We calculated the electronic thermal conductivity of Co using the Wiedemann-Franz law and show that it increases with pressure both in the solid and liquid states and decreases with temperature in the solid and increases in the liquid state. The pressure dependences of electrical resistivity and electronic thermal conductivity calculated from equations involving bulk modulus and the Gruneisen parameter are in reasonable agreement with values measured in this study. The constant resistivity of Co along its melting boundary found in our study portends similar behavior for its electronic structural analog, Fe. This prediction suggests that the electronic thermal conductivity of Fe at Earth's inner core boundary could be similar to its 1 atm value at the melting point. Using this value of thermal conductivity for the inner core boundary would admit thermal convection as an energy source for the geodynamo prior to the birth of the inner core.