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

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-F ₁-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 Fe ₂₁, located as far as at 7.5 Å, is observed due to the 'superfocusing' effect within the Fe₀-F₁-Fe ₂-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).