- Mykhaylo Paukov: Magnetic properties of UH3 -based hydrides.
- 8. 4. 2015, 14:50
- lecture room F2, first floor Ke Karlovu 5
- more information
Abstract:
Interaction with hydrogen is an important issue related to application of uranium and its alloys. Any contact of α-U with H leads to formation of the stable allotropic form β-UH3, a dark pyrophoric powder, formed via the metastable α-UH3. Bcc U-Mo and U-Zr alloys form different types of hydrides, although both are based on UH3. The U-Mo alloys lead to amorphous structure based on β-UH3 (grain size 1 nm). They are ferromagnets exhibit first an increase of the Curie temperature with the concentration of alloying element, and TC can exceed T = 200 K.
Starting from the U-Zr alloys and using high pressures of hydrogen gas, we obtained hydrides (UH3)1-xZrx. For low Zr concentrations we have a mixed crystal structure α-UH3 and β-UH3. With increasing x, the α-UH3 phase, which is the bcc U structure filled with H, dominates. We assume that unlike smaller Mo atoms, bigger Zr atoms occupy U positions without problem. Our investigation of the hydrides in the α-phase shows that despite different crystal structures and U-U spacing, the properties of the α- and β-phases are very similar. Even TC exhibits the same type of non-monotonicity, but the values do not exceed 190 K in the Zr case [1]. The coercive force increases with Zr doping up to 6 T for (UH3)0.70Zr0.30, and it is in the same range for the U-Mo hydrides. Besides magnetic properties, we studied specific heat and electrical resistivity. To determine the spontaneous magnetostriction, the temperature dependence of the lattice parameter was measured in the range 10 - 300 K using X-ray diffraction. Certain hints for understanding of properties of the UH3-based hydrides is provided by ab-initio calculations (by Diviš and I. Turek), which indicate large spin and orbital moments, mutually canceling to a large extent. The spin moments are largely reduced in the paramagnetic state, which explains the large magnetostriction effect.
Preliminary results on double-doped hydrides (e.g. both Mo and Zr) reveal a large flexibility of the materials of the UH3 type, which include also hydrides of U6Fe and U6Co, known for decades already.
[1] I. Tkach, M. Paukov, D. Drozdenko, M. Cieslar, B. Vondráčková, Z. Matěj, D. Kriegner, A.V. Andreev, N.-T.H. Kim-Ngan, I. Turek, M. Diviš, and L. Havela: Electronic properties of α-UH3 stabilized by Zr , Phys.Rev. B 91 (2015) 115116