- The industrial importance of the U–Al–Si system stems from the fact that during processing the Al-based alloy (containing Si as impurity), used for the cladding of U (fuel in nuclear reactors), undergoes heat treatment which stimulates diffusion between the fuel and the cladding. One of the possible ways to represent the ternary U–Al–Si phase diagram is the construction of an UAl3–USi3 quasi-binary phase diagram. On the one hand, since the UAl3 and USi3 phases are isostructural, an isomorphous phase diagram is expected; on the other hand, some researchers observed a miscibility gap at lower temperatures. During our study of the UAl3–USi3 quasi-binary phase diagram, a new stable U(Alx,Si1 − x)3 phase was identified. The structure of this phase was determined, using a combination of electron crystallography and powder X-ray diffraction methods, as tetragonal [I4/mmm (No.139) space group], with lattice parameters a = b = 8.347 (1), c = 16.808 (96) A. Its unit cell has 64 atoms and it can be described as an ordered variant of the U(Al,Si)3 solid solution. A Barnighausen tree was constructed using the original U(Al,Si)3 structure as an aristotype.