Abstract
UH3 is the first discovered material with ferromagnetism based purely on the 5f electronic states, known for more than half century. Although the U metal is Pauli paramagnet, the reduced 5f-5f overlap in compounds allows for moment formation and ordering, typically if the U-U spacing exceeds the Hill limit, i.e. about 340 pm.
The stable form of UH3, known as beta-UH3, has rather high T-C approximate to 170 K. Such high value is rather unusual, considering d(U-U)=331 pm.
Properties of metastable alpha-UH3 with d(U-U)=360 pm could be never well established. Using the fact that alpha-UH3 is in fact bcc U with interstitials filled by H, we attempted to synthesize alpha-UH3 starting from the gamma-U alloys, with the bcc structure retained to room temperature by doping combined with ultrafast cooling.
While up to 15% Zr a contamination by beta-UH3 was obtained, 20% Zr yielded single phase alpha-UH3. The Tc value remains high and very similar to beta-UH3.
One can see an increase up to 187 K for 15% Zr, followed by a weak decrease. Magnetic moments remain close to 1 mu(B)/U atom.
An insight is provided by ab-initio calculations, revealing a charge transfer towards H-Is states, depopulating the U-6d and 7s states, leaving almost pure 5f character around the Fermi level. The 5f magnetism exhibits a high coercivity (mu H-0(C) up to 5.5 T) and large spontaneous volume magnetostriction of 3.2*10(-3).
Even higher increase of T-c, reaching up to 203 K, can be achieved in analogous Mo stabilized hydrides, which yield an amorphous structure. The compounds represent, together with known hydrides of U6Fe and U6CO3 a new group of robust 5f ferromagnets with small d(U-U) but high T.
Although common hydrides are fine powders, some of the new hydrides described as (UH3)((1-x))T-x (T=Zr or Mo) remain monolithic, which allows to study transport and thermodynamic properties.