On the body centered structure of iron at Earth Core Conditions

HR: 11:38h
AN: MR22A-07 [Abstracts]
TI: On the body centered structure of iron at Earth Core Conditions
AU: * Jeanloz, R
EM: jeanloz@berkeley.edu
AF: University of California Berkeley, Earth & Planetary Science, Berkeley, Berkeley, CA 94720, United States
AU: Stixrude, L
EM: stixrude@umich.edu
AF: University of Michigan, Department of Geological Sciences, Michigan, 425 E University Avenue, Ann Arbor, Michigan, Michigan, MI 48109-1063, United States
AU: Verma, A
EM: akverma77@yahoo.com
AF: Bhabha Atomic Research Centre, High Pressure Physics Division, Mumbai, MR 400085, India
AU: Modak, P
EM: pmodak@barc.gov.in
AF: Bhabha Atomic Research Centre, High Pressure Physics Division, Mumbai, MR 400085, India
AU: Godwal, B
EM: godwal@calmail.berkeley.edu
AB: The stability of the body-centered cubic (bcc) phase of iron is considered at Earth-core conditions (V = 7.2 Å³, T = 5500 K) by way of ab-initio molecular-dynamics simulations and electronic-structure calculations using the density-functional approach. Tetragonal strain splits a peak in the electronic density of states at the Fermi level, lowering the energy of the structure with distortion and therefore documenting the metastability of the bcc structure. As temperature is increased from zero to 5500 K, the bcc structure satisfies the condition of hydrostaticity, with vanishing of stress anisotropy, but it remains energetically unstable with respect to the tetragonal distortion; an increase in elastic anisotropy is documented with increasing tetragonal strain. An energy minimum is observed around c/a = 0.9 in a Bain-path plot, and has previously been interpreted as evidence of an elastically-stable body-centered tetragonal (bct) structure. However, we find from static total-energy calculations as a function of b/a and c/a that this is a local minimum in energy corresponding to a metastable structure. Analysis of the structure with tetragonal strain reveals the presence of anomalous shear stresses that, coupled with phonon instability, suggest a tendency toward formation of a hexagonal closed-packed (hcp) structure. Overall, these results argue against bcc or even bct structures of Fe being present in Earth's core, and instead highlight the importance of the known hcp (ε) high-pressure phase of iron.
DE: 3900 MINERAL PHYSICS
DE: 3909 Elasticity and anelasticity
DE: 3919 Equations of state
DE: 3924 High-pressure behavior
DE: 3939 Physical thermodynamics
SC: Mineral and Rock Physics [MR]
MN: 2007 Fall Meeting