THE CRYSTAL-STRUCTURE OF YPDSI, THE ISOTYPIC COMPOUNDS LNPDSI (LN = GD-LU), AND THEIR STRUCTURAL RELATION TO SOME OTHER EQUIATOMIC COMPOUNDS OF THE RARE-EARTH AND TRANSITION-METALS WITH MAIN-GROUP ELEMENTS

The nine title compounds were prepared from the elements by are-meltin g and subsequent heat treatment in resistance and high-frequency furna ces. The crystal structure of these isotypic compounds was determined for YPdSi from single-crystal X-ray diffractometer data: Pmmn, a = 430 .8(1)pm, b=1391.2(1)pm, c = 743.1(1)pm, Z = 8, R = 0.024 for 417 struc ture factors and 40 variable parameters. The crystal structures of the isotypic compounds GdPdSi and ErPdSi were also refined from single-cr ystal data. The structure is of a new type. It consists of condensed, six-membered rings of alternating palladium and silicon atoms with Pd- Si bond distances varying between 249.6 and 258.8 pm. These two-dimens ionally infinite nets are connected to each other via weak Pd-Si and S i-Si bonds with bond distances of 276.3 and 259.5 pm. The rare earth a toms are situated above and below the six-membered palladium-silicon r ings in a manner as it is known for the aluminum atoms in the AlB2 typ e structure. The crystal-chemical similarities and topologies of sever al structures derived from the aristotype AlB2 (including those of BaP tSb, EuAuGe, KHg2, ZrBeSi, and TiNiSi) are described, emphasizing thei r group-subgroup relationships. The previously reported compound ''Er2 Pd2Si'' has the same structure as has been found here for ErPdSi.