Hydrogenation Effects in UTGe Compounds: Structure and Magnetic Properties

UTGe compounds absorb hydrogen up to 2 H atoms/f.u., which has a dramatic impact on the 5f magnetism. The TC value of UCoGe increases to 50 K. Magnetic susceptibility of spin fluctuator UFeGe markedly increases. For ferromagnetic URhGe only α-hydride could be obtained with moderate increase of TC and spontaneous moment. The α-hydride of UCoGe is surprisingly paramagnetic despite of a small volume expansion (0.25 %). Introduction Hydrogenation of UTGe intermetallic compounds is a capable tool used for investigation of their magnetic properties. The basic thermodynamic properties of metal-hydride systems can be assessed from the phase diagrams, which show the formation of the solid solution of hydrogen in intermetallic compounds. The solid solution of a small amount of hydrogen (≤ 0.1 H/f.u. in our case) is called αphase (α-hydride) and characterized by the same type of crystal structure as a pure compound, with a negligible volume expansion. For higher concentration of hydrogen we can obtain a β-phase (βhydride) with the crystal structure modified from the orthorhombic TiNiSi-type for the pure compound to the hexagonal ZrBeSi-type for the hydride. Applied pressure of hydrogen and temperature play a crucial role in hydrogenation process. In some cases the maximum available pressure (150 bar in our setup) is not sufficient to reach pure β-phase and the system ends up in a pure α-phase or in the region where the two phases coexist. Our investigation of UTGe compounds hydrides involved all such cases. Hydrogenation, besides a notable lattice expansion for β-hydrides, leads to the modification of magnetic properties. This fact was already shown in some hydrogenated UTSi compounds (UCoSi-H, UPdSi-H, UNiSi-H) [Miliyanchuk et al., 2004; Kolomiets et al., 2002]. The saturated hydrides of UTSi compounds contain about 2 H atoms/f.u.. This work inspired us to study respective germanides, which can provide larger interstitials and give more favourable hydrogen storage conditions. UTGe compounds crystallize mostly in the orthorhombic TiNiSi-type of structure, space group Pnma. UFeGe undergoes a distortion at 500 K into the monoclinic low-temperature structure. UCoGe originally supposed to be non-magnetic spin fluctuator [Buschow et al. 1990], was recently identified as possible weak ferromagnet, in analogy to URhGe. The values of Curie temperatures were reported as TC = 3 K and 9 K for UCoGe [Huy et al., 2007] and URhGe [Tran et al., 1998; Havela et al., 1998], respectively. Magnetic susceptibility data for UFeGe indicate paramagnetic behaviour with characteristic features of spin fluctuations [Havela et al., 1998]. Experimental details The parent compounds UTGe were prepared by arc melting of stoichiometric amounts of the pure components. The crystal structure was checked by powder X-ray diffraction (XRD-3003 Seifert diffractometer, Cu-Kα radiation). For the hydrogenation the sample was crushed into sub-millimeter pieces and placed into a reactor. Next step is the surface activation by annealing up to T = 523 K under high-vacuum conditions (pressure of 10 mbar). The reactor is subsequently filled by pure hydrogen gas and thermally cycled (twice) to T = 773 K at a given H pressure to promote hydrogen absorption. The main indication of H absorption is a variation of crystal structure parameters detected by XRD. The hydrogen content can be determined by the thermally induced decomposition of the hydride in a closed volume. Magnetic properties were studied both for the UTGe compounds and its hydrides by 105 WDS'09 Proceedings of Contributed Papers, Part III, 105–108, 2009. ISBN 978-80-7378-103-3 © MATFYZPRESS