Eva Šantavá

Are RENiAl hydrides metallic

Abstract Due to hydrogenation, the crystal structure of GdNiAl distorts from hexagonal to orthorhombic. The magnetic ordering temperature decreases dramatically and the magnetic entropy of this phase transition is in good agreement with theory. LuNiAl does not change its structure type, but it was found that the electronic contribution to specific heat decreases dramatically. None of the two phases loses its metallic character.

Low-temperature Properties of U2Co2InH1.9

Abstract U2Co2InH1.9, synthesized by high-pressure hydrogenation of U2Co2In, crystallizes in the tetragonal structure similar to the parent compound, expanded by 8.4 %. Although U2Co2In is a weak paramagnet, its hydride shows properties suggesting a proximity to the magnetic order. Its magnetic susceptibility exhibits a maximum at T = 2.4 K, ascribed to spin fluctuations. Magnetization at low temperatures goes through a metamagnetic transition between 2 - 3 T. The specific heat characteristics, with a pronounced upturn of Cp/T vs. T at low temperatures which can be fitted using an additional −T 1/2 term, resemble the behaviour of U2Co2Sn. The γ coefficient of the electronic specific heat, reaching 244 mJ mol−1 K−2, is gradually suppressed by high magnetic fields.

Hydrogenation affecting the 5f magnetism

In the case of 5f magnetism, the net increase of Tc due to the interstitial doping by hydrogen is generally understood as due to the increase of the density of states at the Fermi level, resulting from 5/band narrowing. The record value of Tc among UTX compound can be achieved in hydrides of U(Co,Fe)Sn series (Tc = 108 K for 15% of Fe). This concept, based on the 5f band formed by periodic array of U sites, does not work for Th doped compounds. In the case of U sublattice diluted by a non-magnetic element the expansion cannot have dominant effect, partly because already doping by non-magnetic Th leads to an expansion.

Effect of hydrogen doping in UTX compounds

Magnetic measurements on the hydrogenated systems U(Co,Fe)Sn and (U,Th)CoSn were performed as part of our investigation of UTX compounds (T—transitional metal and X—p-metal). Both series studied crystallize in the hexagonal structure of ZrNiAl type, space group P6¯2m symmetry. The crystal-structure type is not changed upon the hydrogenation. In all the cases it leads to a unit-cell expansion (ranging between 3.0% and 3.6%). The Curie temperature TC increases in the hydrides. This increment is reduced with increasing Fe concentration. In the compounds with the Th substitution, the Th concentration at which the ferromagnetism vanishes (∼60%) remains practically unchanged in the hydrides.

Hydrogen as a probe into 5f-magnetism

Abstract We performed magnetic measurements on the hydrogenated system (U, Th)CoSn as part of an investigation of UTX compounds (T – transitional metal, X – p-metal). The U1–xThxCoSn compounds crystallize in the hexagonal structure (ZrNiAl type, space group P 2m). Hydrogen absorption of approximately 1 H/f. u. could be achieved. The hydrogenation does not change the crystal structure. In all cases it leads to a unit-cell expansion by 3.0 % to 3.6 %. Studies of the temperature dependence of the magnetic susceptibility indicate a certain increase in the Curie temperature TC in the hydrides, but the Th concentration at which ferromagnetism vanishes remains practically unchanged (U0.2Th0.8CoSn has a paramagnetic ground state).