Heat capacity, neutron diffraction and crystal field models for AnNY (An=Th,U; Y=Se,Te)

New thorium and uranium ternary compounds AnNY (An=Th,U; Y=Se,Te) with tetragonal PbFCl-type crystal structure and metallic conductivity have been synthesized. Neutron diffraction investigations show the absence of magnetic order in UNSe down to 5 K and the presence of a simple collinear ferromagnetic order below 52 K for UNTe with magnetic moment μord=(0.85±0.16) μB aligned along the c axis. Heat capacity measurements are consistent with these conclusions. The electronic low temperature contributions are unusually large: 22 mJ mol-1 K-2 and 60 mJ mol-1 K-2 for UNTe and UNSe respectively. Tentative interpretations based on crystal field models and molecular field approximation are given for the magnetic properties of UNTe as well as for the Schottky contribution to the heat capacity of both uranium compounds. (orig.)