L Journal of Alloys and Compounds 269 (1998) 43–49 Reduced magnetic moments in UNiSi a, b b b c c d * ˇ ˇ ´ K. Prokes , M. Divis , P. Mohn , K. Schwarz ,V. Sechovsky , P. Svoboda , M. Richter , a a a ¨ K.H.J. Buschow , E. Bruck , F.R. de Boer a Van der Waals –Zeeman Institute, University of Amsterdam, Valckenierstraat 65, 1018 XE Amsterdam, The Netherlands b Computational Quantum Theory Group, Technical University Vienna, Getreidemarkt 9 /158, A-1060 Vienna, Austria c Department of Metal Physics, Charles University, Ke Karlovu 5, 12116 Prague 2, Czech Republic d Institute of Theoretical Physics, University of Technology, Mommsenstrasse 13, D-01062 Dresden, Germany Received 19 January 1998 Abstract Powder diffraction experiments reveal that UNiSi crystallises in the orthorhombic TiNiSi-type structure (space group Pnma). Anomalies in the temperature dependence of the magnetisation susceptibility, specific heat and electrical resistivity indicate that UNiSi is magnetically ordered below 87 K. At 18 K, UNiSi undergoes an additional magnetic phase transition. Below this temperature a 21 22 ferromagnetic component develops. The enhanced C / T value extrapolated to 0 K, which amounts to 133 mJ mol K , does not change p in magnetic fields up to 5 T. No magnetic contribution to neutron scattering was detected in neutron powder diffraction pattern experiments down to 2.6 K. Electronic structure calculations were performed using the optimised LCAO method in the local density approximation. Fully relativistic spin-polarised calculations including orbital polarisation of itinerant 5f states yield a magnetic moment of 0.29 m / U and magnetocrystalline anisotropy energies ( E 2 E ) 5 53.6 meV/f.u. and ( E 2 E ) 5 7 meV/f.u. We propose to classify B c a b a UNiSi as an itinerant 5f-electron system with drastically reduced U moments.  1998 Elsevier Science S.A. Keywords: Electrical resistivity; Electronic structure calculations; Magnetisation; Specific heat; Uranium nickel silicide 1. Introduction UNiSi represents one of the ternary equiatomic silicides UTSi (T5late transition metal) [1]. Several crystal struc- Uranium-based compounds have attracted considerable ture types have been proposed for UNiSi, namely the attention over the last few decades because of their CeCu -type [1–3], the hexagonal AlB -type [4,5] and 2 2 intriguing electronic properties. Special attention has been recently the TiNiSi-type (Fig. 1) [6,7]. paid to the general tendency of U to carry a magnetic The basic structural and magnetic properties of UNiSi ´ moment and to the evolution of magnetic ordering. It has have been investigated by Troc and Tran [2] and by Tran been widely accepted that hybridisation effects are one of et al. [3]. On the basis of the X-ray diffraction data they the most important issues in this field. The strong depen- proposed that UNiSi is formed in the CeCu -type structure 2 dence of 5f–ligand hybridisation on the geometry of the where the Ni and Si atoms randomly occupy the 8h environment surrounding a U atom in the solid makes position, resulting in a body-centred orthorhombic lattice. comparative studies of isostructural compounds highly From the temperature dependence of the magnetic suscep- desirable, especially when they are performed on single tibility, they suggested that UNiSi undergoes magnetic crystals. Metallurgical difficulties, however, often prevent phase transitions at 80 and 7.5 K [2]. Some of these results ¨ successful growth of single crystals. In such cases, includ- were confirmed by de Boer et al. [4] and by Bruck [5]. ing UNiSi, experiments must be carried out on poly- The aim of the present investigation was to verify the crystalline samples. magnetic phase transitions and to determine the electronic properties and crystal and magnetic structure of UNiSi at low temperatures. For this purpose, we performed mag- netisation, ac-susceptibility, electrical resistivity and spe- * Corresponding author. Present address: Department of Physics, Facul- cific heat measurements, X-ray and neutron powder dif- ty of Science, Hiroshima University, Higashi-Hiroshima 739, Japan. fraction studies and electronic structure calculations in the Tel.: 181 824 24 7367; fax: 181 824 24 0716; framework of density functional theory. e-mail: prokes@butsuri.sci.hiroshima-u.ac.jp 0925-8388 / 98 / $19.00  1998 Elsevier Science S.A. All rights reserved. PII S0925-8388(98)00254-0