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

 

Chem. Met. Alloys 1 (2008) 352-359

 

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 Pr4Al3.117(3)Ge2.883(3), Mr = 857.01(8), oS20, (63) Cmcmc5, a = 4.161(1), b = 26.261(8), c = 4.373(1) Å, Z = 2, Dx = 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