Structure and magnetic properties of hydrides based on Uranium bcc alloys
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Structure and magnetic properties of hydrides based on Uranium bcc alloys Ladislav Havela,1 Mykhaylo Paukov,1, Ilya Tkach,1 Zdenek Matej,1 N.-T.H. Kim-Ngan2 and Alexander V. Andreev3 1
Faculty of Mathematics and Physics, Charles University, Ke Karlovu 5, 12116 Prague 2, Czech Republic. 2 Institute of Physics, Pedagogical University, Podchorazych 2, 30-084 Krakow, Poland. 3 Institute of Physics, Academy of Sciences, Na Slovance 2, 18221 Prague, Czech Republic. ABSTRACT γ-U alloys with Mo or Zr are more resistant to hydrogen than U metal. High pressures of H are needed to produce hydrides. Amorphous structure of UH3Mox can be represented as the cubic structure of β-UH3 type with grain size around 1 nm. UH3Zrx are formed in the cubic αUH3 type of structure. All the hydrides are ferromagnets, with magnetic parameters (magnetic moments, Curie temperature) exceeding those of β-UH3 (0.9 μB/U, 165-170 K). It is deduced that α-UH3 has magnetic properties very similar to β-UH3, despite rather different U-U spacing. INTRODUCTION Uranium metal exhibits a dramatic reaction with hydrogen leading to the hydride UH3. Its stable variant, β-UH3, is a ferromagnet with magnetic moments μ = 0.9 μB/U [1]. The main reason why the 5f magnetism arises (U metal is a weak Pauli paramagnet) can be seen in the minimum U-U spacing, enhanced from 284 pm to 330 pm. The proximity to the Hill limit, at which the ordering of U moments can set in due to the 5f band narrowing (340 pm), can make difficult to understand the relatively very high Curie temperature TC = 165-170 K. β-UH3 forms via α-UH3 [2], which is metastable and can be partly retained at low temperatures. Its magnetic properties were disputed, in particular it was not clear whether the observed ferromagnetism with TC similar to β-UH3 is intrinsic or due to admixture of β-UH3 [3]. Interesting composition variants of β-UH3 could be obtained by hydrogenation of U6T compounds (T = Mn, Fe, Co, Ni). The obtained hydrides as U6FeH17 [4] or U6CoH18 [5] have their TC values close to that of β-UH3. We have been testing the reaction γ-U alloys stabilized by Mo or Zr to the H2 gas. It turned out that such alloys are much more resistant to H. High pressures (several bar minimum) are needed to hydrogenate. Here we describe results of structure, magnetic and specific heat studies of the hydrides, which can be characterized by the formula UH3Mox and UH3Zrx. Preliminary data on UH3Mo0.18 were published already in [6]. EXPERIMENT AND DISCUSSION New hydrides have been prepared from splats or bulk pieces of UMox and UZrx alloys [7] exposed to elevated pressures of H2 gas at room temperature. At the pressure of 1 bar, U metal
without doping reacts after several minutes long incubation period (needed for diffusion of H through the surface oxide barrier) in a violent reaction, turning material into very fine powder, prone to self-ignition when exposed to air. When testing a splat UMo0.18 (U0.85Mo0.15), exhibiting the bcc structure (γ-U) only, under the same conditions, no reaction was registered after one we
