P 62 ISSRNS 2012: Abstracts / Synchrotron Radiation in Natural Science Vol. 11, No 1 – 2 (2012)
LOCAL STRUCTURE OF IRON CARBIDE NANOPARTICLES
W. Szczerba1∗, H. Riesemeier1, U. Reinholz1, M. Radtke1, A.F. Th¨unemann1, A. Kaupner2, and C. Giordano2
1BAM Federal Institute for Materials Research and Testing, Unter den Eichen 87, 12205 Berlin, Germany
2Max-Planck-Institute of Colloids and Interfaces, Department of Colloids Am M¨uhlenberg 1, 14476 Potsdam-Golm, Germany
Keywords: EXAFS, iron carbide, nanoparticles
∗e-mail : wojciech.szczerba@bam.de
Fe-C nanosystems are in focus of great interest because of their abundance, low cost, large stoi- chiometric range with improved magnetic proper- ties leading to various applications such as sensors and catalyst. Interestingly, amidst all other iron carbides, Fe3C is the oldest iron carbide which is metastable over long period of time and has tune- able properties depending upon the solute atom.
Bulk iron carbide is most commonly found as a phase called cementite (Fe3C) in steel products and cast iron.
Iron carbide (Fe3C) nanoparticles have been investigated by means of x-ray absorption spec- troscopy (XAFS). The experiment has been con- ducted at the iron K -edge at the BAMline at Bessy II in Berlin, Germany. Based on the analysis of the extended x-ray absorption fine structure (EX- AFS) part of the spectra it was found that the nanoparticles have a local structure not resembling the possible bulk structures of iron carbide, neither the orthorhombic Pnma structure nor the hexagonal P6322. However, it could be determined that the iron absorber is surrounded by a close packed-like disordered iron shell at 2.54(3) ˚A being almost ex- actly the lattice constant of austenite (γ-Fe). More- over, the absorbing iron atom has 3 to 4 carbon atoms at a distance of 1.97(4) ˚A in its vicinity.
Figure 1 : Best fit achieved for a cluster model consist- ing of 12 Fe atoms and 4 C atoms around the iron ab- sorber, magnitude FFT (top) and imaginary part (bot- tom). The agreement is in the fitting range (1.2 – 3.2
˚A) excellent.
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