Nuclear Forward Scattering and First-Principles Studies of the Iron-Oxide Phase Fe4O5

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Date/Time:Wednesday, 20 Aug 2014 - Wednesday, 20 Aug 2014
Location:Rooms 18/19 Physics Hall
Channel:Condensed Matter Physics
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Karunakar Kothapalli Geophysical Laboratory, Carnegie Institution of Washington

Fe4O5 is a recently discovered high-pressure-high-temperature iron oxide recoverable at ambient conditions. Ambient temperature and high-pressure structural and magnetic properties of Fe4O5 determined through Nuclear Forward Scattering (NFS) and X-ray diffraction will be presented. The experimental results are combined with first-principles calculations to provide insights into the magnetic properties of Fe4O5. The samples were synthesized in laser-heated diamond anvil cells at a pressure of about 13 GPa and a temperature of about 1500 K. Fe4O5 crystallizes in an orthorhombic structure and has iron atoms arranged in three non-equivalent crystallographic sites. NFS spectra collected in the range 0 - 40 GPa show strong magnetic interactions persisting up to 40 GPa and they are generated by a single magnetic site. The magnitudes of derived hyperfine parameters, the hyperfine magnetic field (Bhf) and the quadruple splitting (QS), suggest that the major contribution to the spectra originates from Fe3+ ions in high spin state. The QS shows an intriguing evolution with pressure with a fast increase between 0 and 10 GPa and a slow increase in the range 10 - 40 GPa. First-principles calculations upto 30 GPa suggest antiferromagnetic ordering in the compound and similar magnetic moments in the range ~3.6 - 3.8 ?B/Fe on all three Fe sites. The single-site contribution to NFS spectrum and the similar calculated magnetic moments suggest that the iron ions at the three crystallographic sites have similar electronic arrangements with non-integer valency. X-ray diffraction data up to 15 GPa will be presented to understand the structural details and to complement the NFS results. The experimental results will also be discussed in comparison with other iron oxides, magnetite in particular that is closely related to Fe4O5 through synthesis and breakdown at high pressures and high temperatures. The relevance of a high pressure phase recoverable at ambient conditions and some details of high pressure technique using diamond anvil cells will also be discussed.