Determination of the structure of a new tetragonal U₂FeAl₂₀ phase

The atomic structure of a new ternary U₂FeAl₂₀ phase appearing in the Al-rich corner of a U-Fe-Al system was solved using electron crystallography and X-ray powder diffraction techniques (XRD). The positions of U atoms were determined from crystallographically processed high-resolution electron microscopy (HRTEM) images. These positions were used as a starting set for determining the coordinates of Fe and Al atoms by difference-Fourier synthesis technique. The U₂FeAl₂₀ phase is tetragonal and belongs to the I over(4, ̄) 2 m space group. Its unit cell contains 80 Al, 4 Fe, and 8 U atoms. The lattice parameters obtained after Rietveld refinement are: a = 12.4138 Å, c = 10.3014 Å. The reliability factors characterizing the Rietveld refinement procedure are: R_p = 8.65%, R_wp = 11.2% and R_b = 5.93%.