First-principles calculations of the structural, mechanical, electronic and bonding properties of (CrB2)n CrAl with n = 1, 2, 3

The crystal structure, the electronic structure and the mechanical properties were investigated for (CrB2)nCrAl with n = 1, 2, 3 by using the all-electron projector augmented wave method with the Perdew-Burke Ernzerhof functional. The calculated bulk moduli for Cr2AlB2, Cr3AlB4 and Cr4AlB6 were 202, 222 and 234 GPa, respectively, and the Young moduli were estimated to be 397, 412 and 434 GPa, respectively. All three materials were identified as stiff and hard materials; however, the density of states and the projected density of states analysis predicted a metallic behavior for all investigated materials. The partially filled d-orbitals of Cr atoms have an important role in attributing metallic behavior to these materials. The hardness of these materials was related to the presence of quite strong BB covalent bonds and the metallic characteristic arises from CrCr interactions. The chemical interpretation of the physical properties was made with the electron localization function and the investigation of the topology of the electron densities was provided, based on the quantum theory of atoms in molecules. Surface properties were explored for slab models, arbitrarily chosen to be along the a-axis. The metallic behavior of the compounds (CrB2)nCrAl continues in the surface model. Using the electron localization function and Bader's charge analysis, a small increase of the number of electronic carriers was observed, due to the localization of some AlAl bonds in the slab models.