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

Abstract

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 4(4)-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)2Aly(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.

Atomic structure and lattice defects in nanolaminated ternary transition metal borides

“…The a, b, and c lattice parameters measured from these patterns are 3.20, 13.96, and 3.10 Å, respectively. These results, again, are in good agreement with ours [27] and those of Ade and Hillebrecht [16]. Figure 4(b) shows the nanolaminated MoAlB structure along the [001] and [100] zone axes, respectively, imaged with an A-HRSTEM.…”

“…sequence in MoAlB, the SF has a MoMoAlMoMo sequence, which is the same as the sequence in M 2 AlB 2 . If synthesized it would constitute a novel Mo-Al-B phase structure, not previously reported [16].…”

“…It is fairly well established at this time that some Al-containing MAX phases, notably Ti 2 AlC, are quite oxidation-resistant because Al diffuses out of the structure and forms a dense protective surface aluminum oxide layer [13][14][15]. Our recent work was spurred by the work of Ade and Hillebrecht, who synthesized single crystals of many previously reported MAlB (M = Mo, W) and M 2 AlB 2 (M = Cr, Mn, Fe) compounds, and discussed their structural relationship to other transition metal borides and their similarity to the MAX phases [16]. Isostructural M-site solid solutions, namely (Mo x , Me 1−x )AlB, where Me = Cr, W and (Fe x , Me 2−x )AlB 2 , where Me = Cr, Mn, have also been reported [17,18].…”

Electrical conductive and damage-tolerant nanolaminated MAB phases Cr2AlB2, Cr3AlB4 and Cr4AlB6

“…Formation of nanolaminated structures like MAX phases, a family of layered ternary carbides and nitrides with a general formulae of M n + 1 AX n (where M is an early transition metal, X is carbon or nitrogen, A is a III A -VI A group element, n = 1-6) [6][7], has been proven to be an effective way to overcome the brittleness of transition metal carbides and nitrides. Inspired by the success in development of MAX phases, nanolaminated ternary borides called MAB phases (where M is a transition metal, A is a III A or IV A group element and B is boron) were recently synthesized and characterized by Ade and Hillebrecht [8] and the stacking sequence and defects were observed by Lu et al [9]. It is expected that MAB phases are damage tolerant, resistant to thermal shock and machinable like MAX phases.…”

“…It is expected that MAB phases are damage tolerant, resistant to thermal shock and machinable like MAX phases. However, besides crystal structure and microhardness [8,9], the properties of many MAB phases are not available. Quantum mechanical modeling through first-principles calculations based on density functional theory (DFT) has been proven a powerful tool to predict the intrinsic properties of transition metal borides [10].…”