  U-based binary alloys have been widely adopted in fast nuclear reactors, but
their stability under extreme conditions of high-pressure is almost unknown,
mounting up to latent risk in applications. Here, possible ordered phases in
U-Zr system up to 200 GPa are comprehensively investigated by unbiased
first-principles structure prediction. Stable U2Zr, metastable U3Zr and U4Zr
phases are discovered for the first time, which exhibit strong stability under
compression. They all are metallic, with 5f electrons of uranium dominating the
electronic density of states near the Fermi level. Prominent ionic interactions
between U and Zr atoms, as well as covalent interactions between adjacent
uranium atoms, are found. The same strategy is applied to explore the stability
of ordered phases in other U-based binary transition metal alloys, U-X (X= Sc,
Ti, V, Cr, Y, Nb, Mo, Hf, Ta, W). Stable and metastable ordered phases similar
to U-Zr alloy are unveiled, all with similar electronic structures. For these
alloys, we find that the structure of U2X (X=Zr, Ti, Hf) hosts a unique hybrid
phase transition similar to U2Nb, which is a superposition of a first-order
transition and a second-order transition. The prediction of these novel phases
not only refutes the stability of the long-believed ordered phase I4/mmm-U2Mo,
but also rewrites the phase diagrams of U-X (X= Zr, Sc, Ti, V, Cr, Nb, Mo, Hf,
Ta) alloys under high pressure. All of these findings promote our understanding
of the high-pressure behavior of the broad category of U-based binary alloys
with transition metals.
