Structural instability and ground state of the U2Mo compound

Abstract

This work reports the structural instability at T=0 • K of the U 2 Mo compound in the structure C11 b under the distortion related to the C 66 elastic constant. The electronic properties of U 2 Mo such density of states (DOS), bands and Fermi surface (FS) are studied to understand the source of the instability. The C11 b structure can be interpreted as formed by parallel linear chains along the z-directions each one composed by successive U-Mo-U blocks. The hybridization due to electronic interactions inside the U-Mo-U blocks is slightly modified under the D 6 distortion. The change in distance among chains modifies the U-U interaction and produces the split of f-states. The distorted structure is stabilized by the energy lowering of the hybridized states, mainly between d-Mo and f-U states, together with the f-band split. Consequently, an induced Peierls distortion is produced in U 2 Mo due to the D 6 distortion. In addition, the results of this work remarks that the structure of the ground state of the U 2 Mo compound is other than the assumed C11 b structure.

On the ground state of the U-Mo system

“…Sometimes, a tiny distortion of atoms positions can lead to different space group symmetries. This is what happened in the case of the U 2 Mo Ω-phase where the structure with the P6/mmm space group turned out to be only 0.1 mRy more stable than the Pmmn structure in the U 2 Mo compound [20,22,23] . 2) The modeling method selected in the DFT code should properly describe the physics of the system; otherwise, the structure could evolve to a situation different to the naturally occurring one.…”

“…The Ωphase has recently been studied in detail [20][21][22] . Nowadays, it is assumed to be the ground state for the U 2 Mo compound as it proved to be more stable than the previously considered one with I4/mmm space group.…”

“…Furthermore, the existence of a new structure with the hexagonal P 6/mmm space group (coincidentally resembling the structure of δ-UZr 2 phase) was predicted by using theoretical ab initio calculations. It was called Ω-phase and it should show up in the phase diagram at least in the vicinity of absolute zero as it is ∼ 4.7 mRy more stable than the I4/mmm, previously assumed as the GS for this composition [20,21] . The total energy of both structures were computed for the sake of comparison in the primitive unit cell.…”

“…However, it was recently found that the U 2 Mo composition with the I4/mmm structure is unstable upon applying a distortion related to the C 66 elastic constant [20,21] . Furthermore, the existence of a new structure with the hexagonal P 6/mmm space group (coincidentally resembling the structure of δ-UZr 2 phase) was predicted by using theoretical ab initio calculations.…”