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

Abstract

The structure of praseodymium aluminum germanide Pr 4 Al 3 Ge 3 has been determined by single-crystal X-ray diffraction. The refined composition is Pr 4 Al 3.117(3) Ge 2.883(3) , M r = 857.01(8), oS20, (63) Cmcm-c 5 , 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 R 4 Al 3 Ge 3 , where R = Ce, Nd, or Sm. This new structure type can be considered as an intergrowth of slabs cut from three simple structure types: AlB 2 , its ternary substitution variant LiBaSi, and W, following the sequence-LiBaSi-AlB 2-LiBaSi-W-, with eight slabs in the translation unit. The axes of the trigonal prisms in the AlB 2-type slab are rotated by 90° with respect to those in the neighboring LiBaSi-type slabs, as in the α α α α-ThSi 2 type and the intergrowth structure TbGe 2. The structure of Pr 4 Al 3 Ge 3 is also related to the structures of binary borides V 2 B 3 , Ta 3 B 4 , V 5 B 6 , and CrB, and the ternary aluminum germanide Ba 3 Al 2 Ge 2 , which altogether form a structural series with different ratios of intergrown AlB 2-(LiBaSi-) and W-type slabs. Alumogermanide / Rare-earth metal / Single-crystal X-ray diffraction / Crystal structure / Structural series