First-principles study of the structural and elastic properties of Cr2AlX (X=N, C) compounds

The structural, electronic and elastic properties of Cr2AlX, with X=N, C, have been investigated at the density functional theory level by applying a plane-wave pseudopotential approach. The band structure and density of states reveal the metallic features of Cr2AlX. The total and projected density of states indicate that the bonding is achieved through a hybridization of Cr 3d states with Al and X-atom p states. The Cr 3d–X2p bonds are lower in energy and are stiffer than Cr 3d–Al 3p bonds. The charge density distributions indicate that there exist soft Cr—Al and relatively strong Cr–X covalent bonds, which might be responsible for their hardness. The elastic constants were obtained in the pressure range 0–100 GPa, and satisfy the stability conditions for hexagonal crystal, which indicates that these two compounds are stable in the pressure regime studied. By analyzing bulk modulus to shear modulus ratio and Cauchy pressure, Cr2AlC is predicted to be brittleness and Cr2AlN is ductile. The Debye temperature was obtained from the average sound velocity.

The heterogeneity of chemical bonds in Cr2AlX (X=N, C) is observed: soft Cr–Al and relatively strong Cr–X covalent bonds might be contributed to their hardness.Figure optionsDownload as PowerPoint slideHighlights
► Cr 3d–X2p (X=N, C) bonds are lower in energy and stiffer than Cr 3d–Al 3p bonds for Cr2AlX.
► The hardness of Cr2AlX might be ascribed to soft Cr–Al and relatively strong Cr–X covalent bonds.
► The predicted brittleness of Cr2AlC and ductility of Cr2AlN originated from their novel structure.