STRUCTURAL CHEMISTRY AND MAGNETIC-BEHAVIOR OF BINARY URANIUM SILICIDES

Binary uranium silicides have been thoroughly reinvestigated with respect to crystal chemistry and magnetic properties; for most compounds, magnetic studies have been carried out at low temperatures for the first time (2 to 300 K, 0 to 5 T). Formation of all known binary silicides has been confirmed: U3Si, U3Si2 (U3Si2-type), USi (USi-type), U3Si5 (defect AlB2-type), USi1.88 (defect ThSi2-type) and USi3 (Cu3Au-type). At the composition U3Si∼5 three different phases have been observed: the defect AlB2-type as well as a phase separation into two orthorhombically distorted Alb2-type related phases. USi2-x with the tetragonal defect ThSi2-type structure at its silicon poor phase boundary was found to be in equilibrium with USi2-x of the orthorhombic defect GdSi2 type. Precise U-U distances have been derived from X-ray single crystal counter data for U3Si2, USi, for the AlB2-type subcell of U3Si5, and for USi1.84 (defect ThSi2-type). From susceptibility measurements, a band type paramagnetism was observed for U3Si2, whereas a temperature independent paramagnetism was confirmed for USi3. Curie-Weiss paramagnetism was encountered within the investigated temperature range for U3Si5 and USi1.88. A modified Curie-Weiss law was revealed for USi with the USi-type, whereas ferromagnetic ordering at Tc = 125 K was observed for the oxygen stabilized "USi" with the FeB-type. Magnetism of the uranium silicides is discussed as a function of the distant dependent 5f-5f electron overlap and d-f electron hybridization. No superconductivity was observed above 2 K. © 1992.