HR: 16:45h
AN: MR34A-04    [Abstracts]
TI: An Ab Initio Study of Iron at Intermediate Pressures
AU: * Eimer, B C
EM: beimer@nmsu.edu
AF: New Mexico State University, Box 30001, MSC 3D, Las Cruces, NM 88003-8001 United States
AU: Kiefer, B
EM: bkiefer@nmsu.edu
AF: New Mexico State University, Box 30001, MSC 3D, Las Cruces, NM 88003-8001 United States
AU: Steinle-Neumann, G
EM: g.steinle-neumann@uni-bayreuth.de
AF: Bayerisches Geoinstitut, Univ of Bayreuth, Bayreuth, 95440 Germany
AB: It has been long been implied that Earth's core is made up of primarily iron. Ab initio calculations have been used extensively in recent years to investigate physical properties of iron at relevant conditions of pressure and temperature, because these are currently difficult to achieve and control by experiment. However, a number of discrepancies between theory and experiment remain unresolved. Prominent among those are large differences in the room temperature equation of state and single crystal elastic properties at intermediate pressures (15-40 GPa). It is therefore important that an accurate, yet efficient methodological framework is established, able to simulate iron under a wide range of pressures and temperatures. Here we perform first principles computations on iron using the Projected Augmented Wave (PAW) method as implemented in the plane wave code VASP. The electronic exchange and correlation energy is treated within the generalized gradient approximation (GGA). The static equation of state, elastic constants, and Raman modes are calculated for a number of different phases in the pressure range of 0-400 GPa. We find excellent agreement with previous all-electron first principles results confirming that the PAW method provides a reliable tool for high pressure computations on iron. We find that an orthorhombic, anti-ferromagnetic (o-afm) distortion of the hexagonal close packed (hcp) structure, previously described in the literature, to be stable. Preliminary results also indicate that magnetic moments remain finite and stable when electronic thermal effects to room temperature are taken into account. Structural distortions are less than one percent, potentially below the resolution limit of current experiments. However, the effect on macroscopic properties of these small distortions can be disproportionably large. For iron we find that the o-afm structure agrees much better with experimental observations as compared with the non-magnetic hcp phase, examples are the equation of state and single crystal elastic properties. Based on these findings a re-examination of experiments in terms of the reduced symmetry structure may be warranted.
DE: 1015 Composition of the core
DE: 3909 Elasticity and anelasticity
DE: 3924 High-pressure behavior
DE: 5109 Magnetic and electrical properties (0925)
DE: 8124 Earth's interior: composition and state (1212, 7207, 7208, 8105)
SC: Mineral and Rock Physics [MR]
MN: Fall Meeting 2005