Pr₄Al₃Ge₃, a new member of the structural series with linear intergrowth of AlB₂- and W-type slabs

Chem. Met. Alloys 1 (2008) 352-359

https://doi.org/10.30970/cma1.0076

Evgen GLADYSHEVSKII, Nataliya SEMUSO, Roman GLADYSHEVSKII, Karin CENZUAL, Jean-Louis JORDA

The structure of praseodymium aluminum germanide Pr₄Al₃Ge₃ has been determined by single-crystal X-ray diffraction. The refined composition is Pr₄Al_3.117(3)Ge_2.883(3), M_r = 857.01(8), oS20, (63) Cmcm–c⁵, a = 4.161(1), b = 26.261(8), c = 4.373(1) Å, Z = 2, D_x = 5.96 Mg m^-3, R = 0.012, wR = 0.015 for 304 independent reflections with F > 3σ(F). Four sites in Wyckoff position 4c are occupied by Pr(1), Pr(2), Al, and Ge, respectively, and a fifth position 4c by a statistical mixture of Al and Ge with a ratio close to 1:1. The compound is characterized by a homogeneity range extending from 26.6 to 30.6 at.% Ge at a constant Pr content of 40 at.%. Isotypic compounds have been found for R₄Al₃Ge₃, where R = Ce, Nd, or Sm. This new structure type can be considered as an intergrowth of slabs cut from three simple structure types: AlB₂, its ternary substitution variant LiBaSi, and W, following the sequence -LiBaSi-AlB₂-LiBaSi-W-, with eight slabs in the translation unit. The axes of the trigonal prisms in the AlB₂-type slab are rotated by 90° with respect to those in the neighboring LiBaSi-type slabs, as in the α-ThSi₂ type and the intergrowth structure TbGe₂. The structure of Pr₄Al₃Ge₃ is also related to the structures of binary borides V₂B₃, Ta₃B₄, V₅B₆, and CrB, and the ternary aluminum germanide Ba₃Al₂Ge₂, which altogether form a structural series with different ratios of intergrown AlB₂- (LiBaSi-) and W-type slabs.

Substructures formed by Al and Ge atoms in the structures of Pr₄Al₃Ge₃ and Ba₃Al₂Ge₂.

Keywords

Alumogermanide / Rare-earth metal / Single-crystal X-ray diffraction / Crystal structure / Structural series