Lanthanoid Rhenium Aluminides with a High Content of Aluminum: LnRe(2)Al(10) (Ln = Ho-Lu) with a New Structure Type and NdRe(2)Al(10) with CaCr(2)Al(10)-Type Structure.

Abstract

The title compounds were obtained by reaction of the elemental components with an excess of aluminum after dissolving the matrix in hydrochloric acid. The compounds LnRe(2)Al(10) (Ln = Ho-Lu) crystallize with a new structure type, which was determined from single-crystal X-ray data of LuRe(2)Al(10): Cmcm, a = 929.1(1) pm, b = 1027.7(2) pm, c = 2684.1(5) pm, Z = 12. The two different lutetium atoms of the structure are coordinated by four rhenium and 16 aluminum atoms, while the two rhenium positions are in distorted icosahedral coordination of two lutetium and 10 aluminum atoms. The 12 different aluminum atoms have between 12 and 14 neighbors, of which two are rhenium and one or two are lutetium atoms. The structure may be considered as a stacking variant of the recently determined structure of YbFe(2)Al(10). Both structures are related to that of ThMn(12). They contain hexagonally close-packed, puckered sheets similar to those known from the close-packed TiAl(3)-type structure. In LuRe(2)Al(10) and YbFe(2)Al(10) these sheets of the compositions Re(2)Al(6) and Fe(2)Al(6), respectively, alternate with less densely packed layers of the composition LnAl(4). The crystal structures of YbRe(2)Al(10) (LuRe(2)Al(10) type, a = 930.7(2) pm, b = 1029.3(2) pm, c = 2687.9(5) pm) and NdRe(2)Al(10) (CaCr(2)Al(10) type, a = 1293.7(1) pm, c = 517.4(1) pm) were also refined from single-crystal X-ray data. The refinements of the occupancy factors of these as well as those of 20 other intermetallic compounds with high aluminum content indicate a seemingly slightly lower (on average 2-3%) scattering power for the aluminum positions in all cases. This is ascribed to the differing electron distributions in the free and in the bonded aluminum atoms. The cell volume of YbRe(2)Al(10) indicates a mixed or intermediate +II/+III valence of the ytterbium atoms.