Synthesis and crystal structures of the new ternary borides Fe3Al2B2and Ru9Al3B8and the confirmation of Ru4Al3B2and Ru9Al5B8−x(x≈2)

Abstract

AbstractSingle crystals of the new ternary borides Fe3Al2B2and Ru9Al3B8were obtained from the elements at 1900°C. Both compounds represent new structure types which combine well-known features of binary and ternary borides of transition metals in combination with aluminum. The crystal structure of Fe3Al2B2(P2/m,Z=2,a=5.724,b=2.857,c=8.723 Å, β=98.57°) contains tetramers of face-sharing trigonal prisms BFe6with a B4unit in trans-configuration. The tetrameric units are separated by Al-atoms which occupy all remaining rectangular sites of the trigonal prisms. The structure can be derived from Fe2AlB2by insertion of additional FeAl fragments in abccarrangement. The crystal structure of Ru9Al3B8(P6̅2m,Z=1,a=9.078,c=2.913 Å) combines zig-zag chains of boron atoms made of face-sharing trigonal prisms BFe6and isolated BFe6units. Three of these chains are connected by common corners to rods running in direction [001]. The rods are linked to a three-dimensional framework by isolated prisms via common edges. Again, Al occupies the capping positions of the trigonal prisms. Ru9Al3B8is the second representative for the combination of boron zig-zag chains and isolated B atoms. The existence of Ru4Al3B2(P4/mmm,Z=2,a=8.515,c=2.924 Å) and Ru9Al5B8−x(P4/m,Z=1,a=8.741,c=2.923 Å) were confirmed and the crystal structures refined. High quality data reveal a stoichiometric composition for Ru4Al3B2, while in Ru9Al5B8−xthere is a significant underoccupation (i.e. x≈2) of the central boron site within the B4units. The crystal structures of all four compounds represent examples for the combination of CsCl and AlB2fragments as they were frequently found for ternary borides of transition metals.