Impact of Hydrogen Absorption on Electronic Properties and Crystal Structure of U-Alloys

INTRODUCTION

Properties of Uranium hydrides have not been well established despite decades of research. The reason is that normally they are obtained as a fine pyrophoric powder. UH₃ is ferromagnetic with T_C ≈ 165 K [1]. It exists in two different cubic modifications. Besides the stable β-UH₃ phase, there is a metastable form α-UH₃.

RESULTS

We have been studying whether the structure and composition can be varied while preserving the single-phase character. One of the possible routes is to start from γ-U (bcc) structure alloys. One type of structure modification happens for U-Mo alloys, giving hydrides (UH₃)_1-xMo_x, at which X-ray diffraction pointed to lost crystallinity [2]. However, the total scattering study (PDF) revealed the β-UH₃ structure type with grains size of 2-3 nm [3]. Trying to exclude Mo segregation on the nano-scale, high-resolution TEM was employed. This work indicated no Mo segregation.

Alloying with Zr leads to a crystalline hydride [2], but the structure corresponds to the α-UH₃ type. It represents the bcc phase with interstices filled by hydrogen, with volume expanded by 75% (d_U-U = 360 pm).

Another route is in-situ preparation of (U_1-xT_x)-H films (T = d-element) by means of reactive sputtering in H₂ containing atmosphere. The oxidation of the surface can be prevented by sputtering the d-element layer on the top of the hydride. The ex-situ XRD study showed that the sputter deposition leads to the β-UH₃ type of hydride which is highly textured and exhibits large compressive residual strain.

Irrespective of composition and structure, magnetic properties are quite similar. One has to conclude that the U-H interaction is modulating magnetic properties more than the U-U spacing, being the source for the 5f delocalization. Hydrogen in a compound with strongly electropositive elements behaves rather as an acceptor of electrons. Therefore one could expect certain reduction of the 5f occupancy. Calculations using the FPLO method [4] revealed that the 5f occupancy may even somewhat increase, but the 6d and 7s states are depleted. This suggests that the important hybridization of the 5f and 6d states is reduced in the hydrides. The 5f-states may remain practically alone at the Fermi level, which contributes to the 5f band narrowing (incipient localization), and electrical conductivity is reduced.

XMCD measurements on (UH₃)_1-xMo_x and (UH₃)_1-xZr_x powders revealed several differences in the magnetic properties between these two hydrides. Whereas an ordered magnetic moment was found on Mo atoms, Zr atoms seem not to acquire a moment. The measured magnetic moments on Uranium atoms in the samples also differ by about ~0.2 μ_B and the unoccupied part of the 5f band appears to be narrower for the Mo alloyed sample.

CONCLUSIONS

U-Mo and U-Zr alloys form hydrides with stoichiometry analogous to UH_3. These hydrides are stable and do not ignite on air, which allowed investigation of electronic and magnetic properties. Despite rather different crystal structure and inter-U spacing, the electronic properties of α-UH₃ phase are very similar to β-UH₃. The weak dependence of properties on structure and composition hints to importance of U-H interaction, leaving the traditional major tuning parameter, U-U spacing, as secondary.

REFERENCES

[1] R. Troc and W. Suski, J. Alloys Comp. 219, 1, (1995).

[2] M. Paukov et al. Journal of Science: Advanced Materials and Devices 1, 185 (2016).

[3] L. Havela et al., MRS Advances (2016).

[4] L. Havela et al. J. Magn. Magn. Mat. 400, 130 (2016).