AL69TA39 - A NEW VARIANT OF A FACE-CENTERED-CUBIC GIANT-CELL STRUCTURE

The structure of Al69Ta39, cF432, a = 1915.3(10) pm, F43mBAR, Z = 4, 38 variables, was determined from 523 X-ray intensities of a single crystal with I > 3sigma(I) and refined to wR(1) = 0.050. Al69Ta39 represents a new structure type composed of four compositionally and three topologically distinct metal atom clusters as present in alpha-Mn-, gamma-brass- and Cd45Sm11-type structures. The compositions of the clusters range from Al10Ta16 to Al23Ta. Deviations from the crystallographic composition A172Ta39 are due to 50% occupation of one Al site. Eleven out of 17 primary coordination polyhedra are of the Frank-Kasper type. The remaining polyhedra can also be considered as being exclusively terminated by triangulated faces. They are either of b.c.c. type or they combine features of both Frank-Kasper and b.c.c. coordinations.