HR: 1340h
AN: MR43A-0874    [Abstracts]
TI: Crystal Structure and Magnetic Properties of Fe$_{3}$C at High Pressures and High Temperatures
AU: * Prakapenka, V B
EM: prakapenka@cars.uchicago.edu
AF: Consortium for Advanced Radiation Sources, University of Chicago, Chicago, il 60637 United States
AU: Shen, G
EM: shen@cars.uchicago.edu
AF: Consortium for Advanced Radiation Sources, University of Chicago, Chicago, il 60637 United States
AU: Sturhahn, W
EM: sturhahn@aps.anl.gov
AF: Advanced Photon Source, Argonne National Laboratory, Argonne, il 60439 United States
AU: Rivers, M L
EM: rivers@cars.uchicago.edu
AF: Consortium for Advanced Radiation Sources, University of Chicago, Chicago, il 60637 United States
AU: Sutton, S R
EM: sutton@cars.uchicago.edu
AF: Consortium for Advanced Radiation Sources, University of Chicago, Chicago, il 60637 United States
AU: Uchida, T
EM: uchida@cars.uchicago.edu
AF: Consortium for Advanced Radiation Sources, University of Chicago, Chicago, il 60637 United States
AB: Fe$_{3}$C has been suggested as a major phase of the Earth's inner core. A number of experimental studies of equation of state at room temperature led to a density of this material at inner core pressures in excellent agreement with the PREM. However, first-principles computer simulations and measurements of thermal expansion of Fe$_{3}$C below and above ferromagnetic phase transition by neutron powder diffraction at ambient pressure have suggested that the physical properties of Fe$_{3}$C are incomparable with the probable range of the inner core density determined from seismic data. Disparate conclusions might be a result of fact that these studies were conducted at either at room temperature or at ambient pressure. In this work, the crystal structure and physical properties of the cementite phase of Fe$_{3}$C have been studied in situ at high pressures and temperatures in diamond anvil cell (DAC) with x-ray diffraction techniques and nuclear resonant spectroscopy. The Fe$_{3}$C samples were synthesised in two ways: ex-situ in a large volume press at 4 GPa and 1600K with further grinding before loading in DAC, and in-situ in a diamond anvil cell at high temperature and different pressures from a mixture of Fe and C powders. The $^{57}$Fe enriched sample was used for nuclear resonant spectroscopy. On compression at room temperature up to $\sim$60 GPa, a second order ferromagnetic transformation was observed in the pressure range of 20-30 GPa. On decompression, the reverse transformation to the magnetic state appeared at much lower pressures: 10-6 GPa. The crystal structure of Fe$_{3}$C remained orthorhombic (space group {\it Pnma}, Z=4) in the entire pressure range studied. Heating samples at $\sim$1800 K significantly affected the magnetic behavior of Fe$_{3}$C at high pressures. For example, the ferromagnetic transition pressure was reduced by $\sim$10 GPa. In situ x-diffraction at high pressures and high temperatures allowed us to structurally map the phase diagram of Fe$_{3}$C and its melting curve. The implications of these results to the composition of the Earth's inner core will be discussed.
DE: 3924 High-pressure behavior
DE: 3929 NMR, Mossbauer spectroscopy, and other magnetic techniques
DE: 1015 Composition of the core
SC: Mineral and Rock Physics [MR]
MN: 2004 AGU Fall Meeting