Reduced magnetic moments in UNiSi

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 ferromagnetic component develops. The enhanced Cp/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 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 μB/U and magnetocrystalline anisotropy energies (Ec-Ea)=53.6 meV/f.u. and (Eb-Ea)=7 meV/f.u. We propose to classify UNiSi as an itinerant 5f-electron system with drastically reduced U moments. (orig.)