The case for a body-centered cubic phase (α') for iron at inner core conditions

Details The case for a body-centered cubic phase (α') for iron at inner core conditions The case for a body-centered cubic phase (α') for iron at inner core conditions

Oct 1997

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997pepi..103...55m&link_type=abstract

Physics of the Earth and Planetary Interiors, v. 103, p. 55-62.

Physics

25

Scientific paper

The molecular dynamics (MD) method is used to simulate the structures and elastic constants of liquid and solid iron at Earth's core conditions. The configurational energy of the iron system is approximated as a sum of Morse-type pair interactions between Fe ions. The contribution to pressure from the electronic thermal energy is taken from band structure calculations reported for solid iron. The three energy parameters for the Morse potential are determined empirically to reproduce (1) the volume compression data of hexagonal close-packed (hcp) iron up to 304 GPa at 300 K, (2) the density ρ, bulk modulus KT, and volume thermal expansivities α of liquid iron at 1900 K, (3) the three elastic constants of face-centered cubic (fcc) iron at 1428 K, and (4) the α values of both fcc and hcp iron at high pressures. Then we apply the MD method with the optimized potential to simulate ρ, KS, μ (rigidity), and α of liquid, fcc, hcp, and bcc (body-centered cubic) iron at pressures up to 400 GPa, and temperatures up to 8000 K. At these P-T conditions, the fcc, hcp and bcc iron phases have very similar simulated values for both ρ and KS, and the fcc and hcp phases also have essentially the same μ values. However, the predicted μ values are found to be much smaller for the bcc phase than for the fcc or hcp phase. We further apply the MD technique to simulate the 200 and 240 GPa phase transitions at high temperatures previously found from shock compression experiments by Brown and McQueen. When we interpret the 200 GPa and the subsequent 240 GPa transitions as the changes from hcp to bcc, and from bcc to liquid, in which the bcc phase and the other phases are considered to be nonmagnetic at these very high-pressure and high-temperature conditions, the MD simulated values of the longitudinal sound velocity drops at the two transitions compare well with the corresponding shock compression data. This prompts us to suggest that the bcc phase, unlike the hcp and fcc phases, duplicates the longitudinal sound velocity in the 200 to 240 GPa region as found by Brown and McQueen. This, in a consequence, suggests possible existence of bcc iron in the inner core conditions.

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