Journal of Alloys and Compounds 383 (2004) 103–107 UCoSiH 0.7 : new representative of UTSiH x hydrides K. Miliyanchuk a,∗ , A.V. Kolomiets a,b , L. Havela a , A.V. Andreev c a Department of Electronic Structures, Charles University, 121 16 Prague, Czech Republic b Institute for Solid Sate Physics, University of Tokyo, Kashiwa, Chiba 277-8581, Japan c Institute of Physics, Academy of Sciences of the Czech Republic, Na Slovance 2, 182 21 Prague, Czech Republic Abstract The crystal structure and magnetic properties of the hydride UCoSiH 0.7 and products of its thermal decomposition were studied. The hydrogenation of UCoSi leads to the modification of the crystal structure: the symmetry increases from orthorhombic (TiNiSi structure type) to hexagonal (ZrBeSi structure type) for UCoSiH 0.7 . UCoSi is a weak paramagnet. The hydrogenation enhances magnetic susceptibility, but does not yield magnetic ordering. The tendency observed for Curie–Weiss (CW) parameters, i.e. the shift of θ P to less negative values and slight increase of effective moments, can be understood as due to a reduction of the characteristic energy of spin fluctuations. © 2004 Elsevier B.V. All rights reserved. Keywords: Hydrides; Uranium intermetallics; Magnetic properties 1. Introduction Hydrogenation is a powerful tool capable of changing both crystal and electronic structure of intermetallic com- pounds. The H absorption modifies the interatomic spacing and electronic density distribution due to H-metal bonding. Strong affinity to hydrogen leads to an appreciable hydrogen absorption capacity in many lanthanide intermetallics. Mag- netic moments of regular lanthanides as a rule remain intact upon hydrogenation, and magnetic studies of hydrides indi- cate mainly the impact on exchange interactions. An excep- tion is valence fluctuators (e.g. CeNiIn [1] and CeNiSn [2]) in which the hydrogenation can stabilise the valence and lead to a magnetic state. Unlike the majority of the 4f systems, intermetallics based on the 5f metals (actinides) have the f-states participating in the metal bonds and forming band states. Hydrogenation influences, therefore, both the stabil- ity and size of magnetic moments and their interactions. Our previous research has been concentrated on large groups of isostructural ternary compounds of the 1:1:1 sto- ichiometry, containing uranium (or rare earths), one transi- tion, and one non transition (p-metal) element. As we were successful in studies of RENiAl–H systems [3], which crys- tallise in the ZrNiAl structure type, we tested the hydrogena- tion of several isostructural UTAl compounds. Unfortunately only UNiAl, known already before, absorbs hydrogen [4–6]. ∗ Corresponding author. E-mail address: chrismil@dove.karlov.mff.cuni.cz (K. Miliyanchuk). Its absorption capacity is reduced already by small substitu- tions of other transition metals for Ni. Therefore, the atten- tion was turned to another group, UTSi, which crystallises in the TiNiSi-type orthorhombic structure. We synthesised the hydrides UNiSiH x and UPdSiH x , with x ≈ 1 in both cases, and showed how magnetic properties are affected by hy- drogenation [7]. Subsequently, other members of the UTSi series were tested for hydrogen absorption capability, but it appeared that UPtSi, URhSi, and URuSi do not absorb hy- drogen. Nonetheless, absorption was detected in UCoSi, and here we report on structure, stability, and magnetic proper- ties of the UCoSi hydride, as compared with UNiSi–H and UPdSi–H. Magnetic properties of the parent compound, UCoSi, were studied earlier by Troc and Tran [8] and de Boer et al. [9]. Both groups report a weakly paramagnetic behaviour, which is undoubtedly mainly due to a strong 5f–3d hybridisation. This is contrasting with the magnetic order found in UNiSi and UPdSi, in which magnetic ordering temperatures in- crease after hydrogenation. Therefore, it would be interest- ing to check whether hydrogenation and associated lattice expansion would not result in establishing of the magnetic order in UCoSi due to a 5f band narrowing. 2. Experimental and results UCoSi intermetallic compound was used as starting ma- terial for hydrogenation. The samples were crushed into 0925-8388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2004.04.016