Focusing and superfocusing effects in X-ray absorption fine structure at the iron K edge in FeF3

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Abstract
We present an ab initio multiple-scattering (MS) analysis of the Fe K-edge x-ray absorption fine structure (XAFS) in FeF3 having a rhombohedrally distorted ReO3-like structure. Good agreement between theoretical and experimental data was found in the energy range 2.5-14.0 Å-1. It is shown that the MS contribution influences strongly the XAFS signals from atoms located beyond the first coordination shell. In particular (i) the XAFS signal from iron atoms Fe2 in the second shell is amplified due to the focusing effect within the Fe0-F1-Fe2 chains, caused by fluorine atoms F1 of the first shell (Fe0 denotes the absorber, and the number is related to the coordination shell); (ii) the contribution from iron atoms Fe21, located as far as at 7.5 Å, is observed due to the 'superfocusing' effect within the Fe0-F1-Fe2-F10-Fe21 chains with FeFFe approximately=153 degrees and FFeF approximately=180 degrees . It was also found that, despite general similarities between FeF3 and ReO3 crystallographic structures, the Fourier transforms of the XAFS signals at the Fe K and Re L3 edges differ significantly beyond the first-shell peak, mainly due to differences in iron and rhenium scattering amplitudes and phases (the rhombohedral distortion becomes important only for outer shells located beyond 4 Å).