REDUCED MAGNETIC-MOMENTS IN UNISI

Powder diffraction experiments reveal that UNiSi crystallises in the o rthorhombic 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 or dered below 87 K. At 18 K, UNiSi undergoes an additional magnetic phas e transition. Below this temperature a ferromagnetic component develop s. The enhanced C-p/T value extrapolated to 0 K, which amounts to 133 mJ mol(-1) K-2, does not change in magnetic fields up to 5 T. No magne tic contribution to neutron scattering was detected in neutron powder diffraction pattern experiments down to 2.6 K. Electronic structure ca lculations 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 magneti c moment of 0.29 mu(B)/U and magnetocrystalline anisotropy energies (E -c-E-a) = 53.6 meV/f.u. and (E-b-E-a) = 7 meV/f.u. We propose to class ify UNiSi as an itinerant 5f-electron system with drastically reduced U moments. (C) 1998 Elsevier Science S.A.