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 r eflections resulting from an ordered distribution of these atoms are observ ed 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-temperat ure (alpha) modification of GdPdGe. In addition, this structure was refined from single-crystal X-ray diffractometer data of CePdGe: Pnma, 2190.2(9) p m, b = 448.4(2) pm, c = 767.3(3) pm, R = 0.019 for 396 structure factors an d 56 variable parameters. The refinement of this structure for alpha -GdPdG e 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) modificatio n of GdPdGe has a YPdSi-type superstructure with a doubled b axis of the su bcell: Pmmn, a = 430.3(I)pm, b = 1409.9(2) pm, c = 757.6(1) pm, R = 0.019 f or 470 F values and 40 variables. In spite of the long-range order of the p alladium and germanium atoms established for the superstructures, the refin ements of the occupancy parameters suggest small amounts of Pd/Ge disorder for all compounds. The crystal chemistry of these compounds is briefly disc ussed. The new germanide Yb,Pd,Ge, was prepared from the elements in a seal ed 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.