Ternary Borides Cr₂AlB₂, Cr₃AlB₄, and Cr₄AlB₆: The First Members of the Series (CrB₂)_nCrAl with n = 1, 2, 3 and a Unifying Concept for Ternary Borides as MAB-Phases

Single crystals of the ternary borides Cr₂AlB₂, Cr₃AlB₄, Cr₄AlB₆, MoAlB, WAlB, Mn₂AlB₂, and Fe₂AlB₂ were grown from the elements with an excess of Al. Structures were refined by X-ray methods on the basis of single crystal data. All compounds crystallize in orthorhombic space groups. In each case boron atoms show the typical trigonal prisms BM₆. The BM₆-units are linked by common rectangular faces forming B–B-bonds. Thus, zigzag chains of boron atoms are obtained for MoAlB, WAlB, and M₂AlB₂ (M = Cr, Mn, Fe); chains of hexagons for Cr₃AlB₄; and double chains of hexagons for Cr₄AlB₆. The same subunits are known for the binary borides CrB, Cr₃B₄, Cr₂B₃, and β-WB, too. The boride partial structures are separated by single layers of Al-atoms in the case of the chromium compounds and double layers for WAlB, i.e., W₂Al₂B₂. All crystal structures can be described using a unified building set principle with quadratic 4⁴-nets of metal atoms. The different compositions and crystal structures are obtained by different numbers of metal layers in the corresponding parts according to the formula (MB)₂Al_y(MB₂)_x. This principle is an extension of a scheme which was developed for the boridecarbides of niobium. Furthermore, there is a close similarity to the group of ternary carbides MAl­(MC)_n, so-called MAX-phases. Therefore, they might be named as “MAB-phases”. The pronounced two-dimensionality and the mixture of strong covalent and metallic interactions make MAB-phases to promising candidates for interesting material properties. All compositions were confirmed by EDX measurements. Additionally, microhardness measurements were performed.