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Abstract:
The crystal structure of Co4Al13 has been investigated using
metallography, X-ray single crystal and powder diffraction methods. A
sample with the composition Co24Al76 has been synthesized in a high
frequency furnace and heat treated at 850-degrees-C to achieve
homogenization. The crystal structure of Co4Al13 belongs to a new
structure type: Pearson's symbol, oP102; space group, Pmn2(1); a =
8.158(1) angstrom, b = 12.342(1) angstrom, c = 14.452(2) angstrom; V =
1455.1(5) angstrom3; Z = 6. The final value of the reliability factor is
0.0618 for 839 symmetry-independent reflections (\Absolute value of F >
4sigma(F)) with isotropic extinction and anisotropic thermal
displacement coefficients. The shortest interatomic distances are those
of Co-Co (2.86 angstrom), Co-Al (2.24 angstrom) and Al-Al (2.24
angstrom). The coordination numbers of the atoms are 8-11 for Co and
9-13 for Al. The structure of Co4Al13 is built from the same structure
segments as Fe4Al13 but differs from that of Fe4Al13 by the stacking
sequence (numeric symbol 1(2) for Co4Al13 and infinity for Fe4Al13). In
both structures, pentagonal ''channels'' occur. The atoms with
pentagonal-prismatic or icosahedral environment are located on the axes
of these channels.
