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
Pr4Al3.117(3)Ge2.883(3),
Mr
= 857.01(8), oS20, (63) Cmcm–c5, 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