Electronic correlations in Fe at Earth's inner core conditions: Effects of alloying with Ni
O. Yu. Vekilova, L. V. Pourovskii, I. A. Abrikosov, and S. I. Simak
Phys. Rev. B 91, 245116 – Published 8 June 2015
Iron is the main component of Earth's inner core. From geochemical and seismic data, it is generally assumed that iron in the Earth's core is alloyed with 10–15 at.% of Ni. The pressure and temperature inside the inner core are estimated to be ∼330–364 GPa and ∼6000 K, respectively. They are too extreme to be easily accessible in modern diamond anvil-cell experiments, so the actual crystal structure is still a matter of debate. From the discovery of the Earth's core, the hexagonal close-packed (hcp) structure has been the primary candidate. However, in the past decade, a number of scientists started to advocate the body-centered cubic (bcc) or face-centered cubic (fcc) Fe phase to be a suitable candidate for Earth's inner core material. Despite a vast number of publications, most theoretical papers dedicated to the crystal structure and properties of Earth's core presumed Fe to be a nonmagnetic metal with insignificant electronic correlations. Nevertheless, even the combined effect of extremely high pressure and temperature typical of Earth's core may not be sufficient to suppress significant electronic correlations in paramagnetic Fe, as was recently shown by us for pure Fe. However, the presence of Ni in Earth's core, often neglected in first-principles studies, may be important. In the present work we have studied the bcc, fcc, and hcp phases of Fe alloyed with 25 at.% of Ni at Earth's core conditions using an ab initio local density approximation + dynamical mean-field theory (DMFT) approach. Our calculations demonstrate that the strength of electronic correlations on the Fe 3d shell is highly sensitive to the phase and local environment. In the bcc phase, the 3d electrons at the Fe site with Fe only nearest neighbors remain rather strongly correlated, even at extreme pressure-temperature conditions, with the local and uniform magnetic susceptibility exhibiting a Curie-Weiss-like temperature evolution and the quasiparticle lifetime, Γ, featuring a non-Fermi-liquid temperature dependence. In contrast, for the corresponding Fe site in the hcp phase, we predict a weakly correlated Fermi-liquid state with a temperature-independent local susceptibility and a quadratic temperature dependence of Γ. The iron sites with nickel atoms in the local environment exhibit behavior in the range between those two extreme cases, with the strength of correlations gradually increasing along the hcp-fcc-bcc sequence. Further, the intersite magnetic interactions in the bcc and hcp phases are also strongly affected by the presence of Ni nearest neighbors. The sensitivity to the local environment is related to modifications of the Fe partial density of states due to mixing with Ni 3d states.
Figure. a) The inverse uniform magnetic susceptibility in the paramagnetic state as a function of temperature for the hcp phase, with the dashed and solid lines corresponding to the Fe1 (six Ni and six Fe nearest neighbors) and the Fe2 (all nearest neighbors are Fe) types, respectively. The error bars show the stochastic Continuous Time Quantum Monte Carlo (CT-QMC) error. (b) The same data for the bcc and fcc phases of the Fe3Ni alloy. Dashed red line corresponds to the Fe1 type of iron atoms, whose nearest neighbors are four Fe and four Ni atoms in the bcc structure. The solid red line shows the Fe2 type of atoms, which are surrounded exclusively by Fe atoms. The dashed-dotted black line shows the Fe atoms of the fcc phase. The error bars show the stochastic CT-QMC error.
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