The order of the palladium and germanium atoms in the germanides LnPdGe (Ln = La-Nd, Sm, Gd, Tb) and the new compound Yb3Pd4Ge4

In previous publications about the seven ternary equiatomic title compounds only the KHg2, (CeCu2)-type subcells with statistical distribution of the palladium and germanium atoms were reported. However, weak superstructure reflections resulting from an ordered distribution of these atoms are observed in the Guinier powder patterns. A. CaCuGe (alpha -YbAuGe)-type structure with a tripled subcell b axis was established from the powder patterns for the compounds LaPdGe, CePdGe, PrPdGe, NdPdGe, SmPdGe, and the low-temperature (alpha) modification of GdPdGe. In addition, this structure was refined from single-crystal X-ray diffractometer data of CePdGe: Pnma, 2190.2(9) pm, b = 448.4(2) pm, c = 767.3(3) pm, R = 0.019 for 396 structure factors and 56 variable parameters. The refinement of this structure for alpha -GdPdGe resulted in a = 2112.3(3) pm, b = 440.5(1) pm, c = 757.7(1) pm, R = 0.022 for 372 F values and 56 variables. The high-temperature (beta) modification of GdPdGe has a YPdSi-type superstructure with a doubled b axis of the subcell: Pmmn, a = 430.3(I)pm, b = 1409.9(2) pm, c = 757.6(1) pm, R = 0.019 for 470 F values and 40 variables. In spite of the long-range order of the palladium and germanium atoms established for the superstructures, the refinements of the occupancy parameters suggest small amounts of Pd/Ge disorder for all compounds. The crystal chemistry of these compounds is briefly discussed. The new germanide Yb,Pd,Ge, was prepared from the elements in a sealed tantalum tube in a high-frequency furnace and its structure was refined from single-crystal X-ray data: Gd,Cu,Ge, type, Immm, 410.7(1) pm, b = 707,0(1) pm, c = 1372.2(3) pm, R = 0.027 for 667 F values and 23 variables. (C) 2000 Academic Press.