STRUCTURE AND PROPERTIES OF f-METAL HYDRIDES

K. Miliyanchuk^a_, A.V. Kolomiets^a,b_, L. Havela^a_, A.V. Andreev^c

^a Department of Electronic Structures, Charles University, Ke Karlovu 5, 121 16 Prague 2,

The Czech Republic

^b Institute for Solid State Physics, University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8581, Japan

^c Institute of Physics ASCR, Na Slovance 2, 182 21 Prague 8, The Czech Republic

Hydrogenation of uranium-based intermetallic compounds, leading to enhanced U-U spacing and modified bonding conditions, is an interesting probe of 5f-electron systems. Our current study is focused on the interaction of the UTSi (T – transition metal atom) compounds, crystallizing in the TiNiSi structure type, with hydrogen. It appears that not all U intermetallics from this series can be hydrogenated under the available laboratory conditions. We succeeded in obtaining the hydrides of UPdSi, UNiSi and UCoSi, whereas UPtSi, URhSi and URuSi do not absorb hydrogen in the pressure range up to p_H2 = 130 bar and temperatures T £ 923 K.

Hydrogenation of UPdSi and UNiSi leads to the absorption of approximately 1.0 H atom/f.u. X-ray powder diffraction analysis of the hydrides indicates an increase of the unit cell volume (6.6% for UPdSiH_1.0, 8.2% for UNiSiH_1.0) [1]. In the most recently synthesized hydride of UCoSi, the volume expands by 6.6%, while the hydrogen content reaches nearly 0.7 atom/f.u. In all the hydrides the lattice symmetry changes from the orthorhombic (TiNiSi structure type, space group Pnma) to the hexagonal one (ZrBeSi structure type, space group P6₃/mmc). Both crystal structures are derived from the AlB₂ structure type. Positions of the H atoms were not determined due to the very small X-ray scattering factor of hydrogen.

Studies of the thermal stability of UCoSiH_x have shown that hydrogen release occurs in two steps at about 490 K and 820 K. The intermediate hydride, obtained after the first step of decomposition, also adopts the ZrBeSi structure, and the hydrogen desorption is accompanied by decrease of the lattice parameter c, while a remains almost constant. The initial TiNiSi-type structure is restored after the second peak of hydrogen release.

The studies of the magnetic susceptibility revealed interesting changes in the magnetic behaviour of the compounds upon hydrogenation.

References

[1] A.V. Kolomiets et al., Phys.Rev.B 66, 144423 (2002)