The Binary System Np-Si and The Magnetic Properties of Np3Si2

In the frame of our systematic re-investigation of actinides binary systems, we have recently reported on the structural chemistry of the complete binary Np-Ge and we are currently studying the Np-Si system. This system is characterised by the existence of 6 different compounds whereas only 3 were already reported in the literature. As shown in the table, the existence and the crystal structure of NpSi3 (AuCu3 type) and of the binary richest in neptunium Np3Si2 (U3Si2 type) were confirmed. The ThSi2 type of compound was also confirmed but with a slight defect on the silicon site, as usually observed in the rare-earth or actinide systems with silicon and germanium. Moreover, the hexagonal A1B2 type of compounds was isolated for the first time, with, as expected, more silicon defects than the ThSi2 type of compounds. The last phase observed in this system was found to crystallize with the NpSi composition with a polymorphic transformation. Depending on the annealing process we observed a primitive orthorhombic FeB type as observed with the rare-earth systems and an original C-centered orthorhombic crystal structure. The physical properties of all these compounds are now under investigations by magnetisation and 237Np Moessbauer measurements. This paper will focus on the magnetic properties of the binary Np3Si2 for which no physical properties were reported