In-plane ordered quaternary M' 4/3 M'' 2/3 AlB 2 phases (i-MAB): Electronic structure and mechanical properties from first-principles calculations.

We have by means of first principles density functional theory calculations studied the mechanical and electronic properties of the so called i-MAB phases, M'4/3M''2/3AlB2, where M'= Cr, Mo, W and M'' = Sc, Y. These phases, experimentally verified for Mo4/3Sc2/3AlB2and Mo4/3Y2/3AlB2, display an atomically laminated structure with in-plane chemical order between the M' and M'' elements. Structural properties, along with elastic constants and moduli, are predicted for different structural symmetries, including the reported R3 m (#166) space group. We find all considered i-MAB phases to be metallic with a significant peak in the electronic structure at the Fermi level and no significant anisotropy in the electronic band structure. The simulations also indicate that they are rather hard and stiff, in particular the Cr-based ones, with a Young's modulus E of 325 GPa for M'' = Sc. The Mo-based phases are similar, with E = 299 GPa for M'' = Sc, which is higher than the corresponding laminated carbides (i-MAX phases).