HEAVY-FERMION AND SEMICONDUCTING PROPERTIES OF THE TERNARY URANIUM-COMPOUNDS U3T3SN4 AND U3T3SB4 (T=NI, CU, PD, PT AND AU)

We present results of magnetic, transport and calorimetric measurements on U3T3X4 with T = Ni, Cu, Pd, Pt and Au and X = Sn and Sb. A heavy-fermion behavior manifests itself in U3T3Sn4, where the electronic specific-heat coefficient increases from 90 mJ/K2 mol U for T = Ni and Pt to about 300 mJ/K3 mol U for T = Cu and Au. We ascribe the mass enhancement to the dehybridization of 5f electrons with the d electrons derived from the T atom. On the other hand, U3T3Sb4 with T = Ni, Pd and Pt shows a semiconducting behavior with a band gap of about 0.2 eV, whereas U3Cu3Sb4 is a metallic ferromagnet with T(c) = 91 K. In these antimonides, 5f electrons are localized because of the weak hybridization with the 5p electrons from the Sb atom. These results indicate that the number of 5p electrons in the present system determines whether the system shows heavy-fermion or semiconducting behavior.