Abstract
Single crystals of two phases in the binary Mo-Al system were obtained
from heat treated aluminum-rich alloys after the dissolution of the
remaining solidified aluminum in hydrochloric acid. The crystal
structures have been investigated using four-circle diffractometer data:
Mo4Al17 was found to be monoclinic (space group C2, a = 9.158(1)
Angstrom, b = 4.9323(8) Angstrom, c = 28.935(5) Angstrom, beta =
96.71(1)degrees, Z = 4), while Mo5Al22 was determined to be orthorhombic
(space group Fdd 2, a = 73.82(3) Angstrom, b = 9.161(3) Angstrom, c =
4.933(2)Angstrom, Z = 8). Three different polyhedrons represent the
atomic environment: a hexagonal cuboctahedron (CN = 12), a defect
cuboctahedron (CN = 11) and a combination from cuboctahedron and
rhombododecahedron (CN = 13). The crystal structures are interpreted in
terms of the ''intergrowth'' concept as being built from slab segments
taken from the closest packing with the composition TAl5, (related to
WAl5, structure type, T - transition metal) and segments taken from the
TiSi2 structure type (composition TAl2). The composition of the unit
cell is described by the formula Mo2m+2nAl10m+4n(m - number of
TAl5-segments and n - number of TiSi2-segments in the unit cell).
