CRYSTAL-STRUCTURE OF THE TERNARY SILICIDE U2RUSI3 - A NEW ORDERED VERSION OF THE HEXAGONAL ALB2-TYPE STRUCTURE

The ternary silicide U2RuSi3 was prepared from the elemental component s in an induction levitation furnace. It was investigated by electron diffraction as well as by X-ray diffractometry on both a single crysta l and a powder. The crystal structure was determined from four-circle diffractometer data and refined to a residual of R = 0.026 for 145 ref lections and 11 variable parameters. It is of a new type with hexagona l symmetry deriving from the AlB2 type: P6/mmm, a = 814.8(2) pm, c = 3 85.5(2) pm, V = 0.2216(2) nm3, Z = 2. The silicon and ruthenium atoms are all surrounded by trigonal prisms of uranium atoms and are ordered in a two-dimensional network. The ternary silicide U2OsSi3 was found to be isotypic with U2RuSi3. At low temperatures, the magnetic suscept ibility of U2RuSi3 tends towards saturation and characterizes a spin f luctuation behaviour.