Tb2Al3Ge3, a New Partly Disordered Structure Type with Al3 Triangles

Using X-ray diffraction it was established that the actual composition of the ternary compound reported as TbAl2Ge is close to Tb2Al3Ge3 (refined Tb2Al3.039(1)Ge3). Differential thermal analysis showed that the compound exists in a narrow temperature range from 618 to 748 degrees C. The crystal structure was determined from X-ray powder and single-crystal diffraction data. An initial model was constructed from CaAl2Si2- and AlB2- (or LiBaSi-) type slabs. A more detailed investigation revealed that, instead of LiBaSi-type slabs (where the small atoms form planar nets), the structure contains ScAuSi-type slabs (corrugated nets), where 1/6 of the Tb atoms have been removed in an ordered way, leading to a rhombohedral superlattice. The final structure represents a new structure type: Pearson symbol hR234-41.06, space group R (3) over barc, a = 7.2432(3), c = 87.998(8) angstrom. Part of the Al atoms in the Tb-deficient ScAuSi-type slabs occupy a site that forms Al-3 triangles. The structure of Tb2Al3Ge3 is thus closely related to the rhombohedral ErNi3Al9 and orthorhombic Y2Co3Ga9 structure types, which contain one-atom thick Er2Al3 (Y2Ga3) layers formed by rare-earth metal atoms and Al-3 (Ga-3) triangles. Compounds isotypic with Tb2Al3Ge3 were found in the systems Y-Al-Ge and Gd-Al-Ge.