The effect of pressure on the electronic and magnetic properties of the antiferromagnetic ͑T N ϳ 43 K͒ narrow gap semiconductor UNiSn has been investigated by 119 Sn Mössbauer spectroscopy and nuclear forward scattering of synchrotron radiation, electrical resistance, and x-ray diffraction. We show that the decrease of the semiconducting gap which leads to a metallic state at p ϳ 9 GPa is associated with an enhancement of T N. At higher pressures, both T N and the transferred magnetic hyperfine field decrease, with a collapse of magnetism at ϳ18.5 GPa. The results are explained by a volume-dependent competition between indirect Ruderman-Kittel-Kasuya-Yosida interaction and the 5f-ligand hybridization.

The pressure-temperature dependence of the electronic and magnetic properties of the compounds U(In 1−x Sn x ) 3 and UNiSn has been investigated by means of high pressure x-ray diffraction, 119 Sn nuclear forward scattering (NFS) of synchrotron radiation and Mössbauer spectroscopy (MS) measurements. We show that pressure has different effects on these systems: while U(In 0.8 Sn 0.2 ) 3 shows some of the typical properties of nearly localized 5f systems, pressure induces the delocalization of the U 5f electrons in U(In 0.6 Sn 0.4 ) 3 and UNiSn with a pressure dependent interplay between RKKY exchange interaction and the consequent collapse of the magnetic order. These results are discussed in terms of hybridization between U 5f electrons and conduction band electrons such as U 6d, Sn/In 5sp and Ni 3d electrons.