UC Santa Cruz

The Ru K, Fe K, and the U L_III absorption edge of URu_2-xFe_xSi₂ were investigated by the EXAFS technique. No change in the electronic structure was found in the U L_III absorption edge as the Fe concentration varied. Tiny changes in the electronic structure were found in the Ru K absorption edge and the Fe K absorption edge in relation to the Fe concentration. However, the variation in these structures are at the limit of our resolution. The distance between U with its nearest U neighbor was found to decrease from 4.12 Å to 4.11 Å as the concentration x of Fe increased from x=0.00 to x=0.20. Position plots of U with its nearest Ru neighbor and nearest Si neighbor indicate a decreases in distance as temperature decreases. Position plots of Fe K edge show evidence for Fe doping on the Ru site acts as a chemical pressure; the Fe-Si distance decreases by 0.05 Å. The Debye-Waller factor was plotted as a function of temperature which led to the finding of excess disorder in the Ru-Si, Ru-U, U-Ru, and U-U pairs. The remaining atomic pairs fit the Debye model reasonably well. The Ru-Si pair showed a significant increase in σ² from 70 K to 25 K, indicating a split Ru-Si peak. The Ru-U, U-Ru, and U-U pairs also showed a slight increase in disorder at low temperature. A distortion model was constructed that followed these results where the ab-plane of the tetragonal structure of URu_2-xFe_xSi₂ distorts from a square to a rectangle, resulting in an orthorhombic structure in the low temperature region. The elongation of the ab-plane is able to happen along either the a-axis or the b-axis since Ru may have two distinct positions due to the splitting of the Ru-Si bond. This orthorhombic distortion does not affect the U-Si or the Ru-Ru distances which agrees with no excess disorder found in their corresponding σ² plots. A resulting mix of this effect on a local scale (few nm) will have the structure appear tetragonal to X-ray diffraction techniques.