Preparation and Crystal Structure of the Equiatomic Rare Earth Palladium Silicides NdPdSi, SmPdSi,ALPHA-GdPdSi, andALPHA-TbPdSi

The title compounds were prepared by arc-melting of the elemental components. Whereas NdPdSi and SmPdSi are already present after the arc-melting,ALPHA-GdPdSi andALPHA-TbPdSi are formed only during the annealing at 800°C. The four compounds crystallize with the recently reportedALPHA-YbAuGe type structure, which was refined for-GdPdSi:Pnma,a= 2108.0(4) pm,b=433.9(1) pm,c=745.6(1) pm,Z=12,R=0.026 for 1447 structure factors and 62 variable parameters. The lanthanoid atoms are situated between two-dimensionally infinite nets of condensed, puckered hexagons formed by alternating palladium and silicon atoms, with Pd–Si distances varying between 251 and 262 pm. In the third dimension these nets are linked via weak Pd–Pd (300 pm), Pd–Si (283 pm), and Si–Si bonds (261 pm). The refinements of the occupancy parameters suggested that ca. 2% of the palladium sites are occupied by silicon atoms and vice versa. The structural relationships between the two modifications of GdPdSi and the structures of AlB2, KHg2, TiNiSi,-UFeGe, EuAuGe, and EuAuSn are discussed.