Abstract
All atomically laminated MAB phases (M = transition metal, A = A-group element, and B = boron) exhibit orthorhombic or tetragonal symmetry, with the only exception being hexagonal Ti2InB2. Inspired by the recent discovery of chemically ordered hexagonal carbides, i-MAX phases, we perform an extensive first-principles study to explore chemical ordering upon metal alloying of M2AlB2 (M from groups 3 to 9) in orthorhombic and hexagonal symmetry. Fifteen stable novel phases with in-plane chemical ordering are identified, coined i-MAB, along with 16 disordered stable alloys. The predictions are verified through the powder synthesis of Mo4/3Y2/3AlB2 and Mo4/3Sc2/3AlB2 of space group R3̅m (no. 166), displaying the characteristic in-plane chemical order of Mo and Y/Sc and Kagomé ordering of the Al atoms, as evident from X-ray diffraction and electron microscopy. The discovery of i-MAB phases expands the elemental space of these borides with M = Sc, Y, Zr, Hf, and Nb, realizing an increased property tuning potential of these phases as well as their suggested potential two-dimensional derivatives.
Highlights
Transition-metal borides and carbides have high melting points and are among the hardest compounds known, which are properties required for both bulk and coating applications
One route to enhancing the materials’ elemental space and the associated range of attainable properties is through alloying. This is exemplified by the recent discovery of inplane chemically ordered MAX phase alloys, coined i-MAX, which through hexagonal symmetry allow for elemental ordering and the incorporation of metals not found in traditional ternary MAX phases.[6−8] A key feature of i-MAX phases is that by employing different etching protocols the resulting 2D MXene can have in-plane chemical or vacancy ordering,[9] which is valuable for catalysis and energy storage.[6,8]
The thermodynamic phase stability of the ternary M2AlB2 phases are evaluated by assuming both polymorphs: orthorhombic Cmmm (Figure 1a) and hexagonal P6̅m2 (Figure 1b) symmetry
Summary
Transition-metal borides and carbides have high melting points and are among the hardest compounds known, which are properties required for both bulk and coating applications. One route to enhancing the materials’ elemental space and the associated range of attainable properties is through alloying This is exemplified by the recent discovery of inplane chemically ordered MAX phase alloys, coined i-MAX, which through hexagonal symmetry allow for elemental ordering and the incorporation of metals not found in traditional ternary MAX phases.[6−8] A key feature of i-MAX phases is that by employing different etching protocols the resulting 2D MXene can have in-plane chemical or vacancy ordering,[9] which is valuable for catalysis and energy storage.[6,8] Similar in-plane chemical order in 3D-layered transition-metal borides (MAB phases) has, to this date, not been found
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