| AVS 60th International Symposium and Exhibition | |
| Actinides and Rare Earths Focus Topic | Tuesday Sessions |
| Session AC+AS+EN-TuA |
| Session: | Actinides and Rare Earths: The Nuclear Fuel Cycle and Critical Materials |
| Presenter: | L. Havela, Charles University, Czech Republic |
| Authors: | L. Havela, Charles University, Czech Republic I. Tkach, Charles University, Czech Republic S. Maskova, Charles University, Czech Republic Z. Matej, Charles University, Czech Republic N.-T.H. Kim-Ngan, Pedagogical University Cracow, Poland A.V. Andreev, Academy of Science of the Czech Republic |
| Correspondent: | Click to Email |
Uranium metal absorbs H readily at low pressures, forming a fine highly pyrophoric powder of β-UH3, which is ferromagnetic with TC ≈175 K. More technologically important than the orthorhombic α-U is the bcc γ-U, known as existing at high temperatures or stabilized by doping to low temperatures.
In the course of the study of fundamental properties of Mo-stabilized bcc U alloys [1] we have been testing their resistance to H. There was no interaction with H2 at ambient pressure and room temperature. However, increasing pressure over approx. 4 bar a slow hydrogenation was observed after several days if incubation. Increasing the H2 pressure makes the hydrogenation faster. The product, in which the H concentration corresponds to the 1U-3H ratio, does not turn into powder, but consist of large brittle fragments with metallic luster. X-ray diffraction indicates that the hydride is amorphous, with a basic structure pattern corresponding to β-UH3, while the grain size is ≈1 nm only.
The hydrides obtained were subjected to studies of magnetic properties, electrical resistivity, and specific heat. The most striking fact is that the Curie temperature can be higher than in β-UH3. It reaches TC = 200 K for UH3Mo0.18, for both lower and higher Mo concentrations it weakly decreases. Also the magnetization values increase from 0.93 μB/f.u. to 1.2 μB/f.u. (both in μ0H = 14 T). At low temperatures a striking high coercivity, reaching 4 T, is seen from magnetization loops. It can be understood as due to a high anisotropy randomly distributed over spatial directions.
The Sommerfeld coefficient of electronic specific heat γ = 29.2 mJ/mol K2 is similar to β-UH3 values (29-33.2 mJ/mol K2 by various authors) and higher that pure UMo0.18 (18.8 mJ/mol K2). Electrical resistivity exhibits the weakly negative slope known from the bcc U-Mo alloys [2] and other strongly disordered systems with a cusp-like anomaly at 200 K superimposed. The absolute values about 1 mΩcm are about an order of magnitude higher than for UMo0.18, and even higher than that of UH3, which exceeds 600 μΩcm at room T [3]. The most surprising fact is that the 5f magnetism, which is normally suppressed by disorder, is supported in the case of UH3Mox, probably due to enhanced U-U spacing due to Mo included in the lattice.
This work was supported the Czech Science Foundation under the grant No. P204/12/0285.
[1] I. Tkach, N.-T.H. Kim-Ngan, S. Mašková, M. Dzevenko, L. Havela, J. Alloys Comp. 534 (2012) 101.
[2] B.S. Chandrasekhar, J.K. Hulm, J. Phys. Chem. Solids 7 (1958) 259.
[3] J.W. Ward, L.E. Cox, J.L. Smith, G.R. Stewart, J.H. Hood, J. Phys. 40 (1979) C4-15.