Physical properties of the recently discovered Zr2(Al1−x Bi x )C MAX phases

In this study we investigate the structural, electronic and optical properties of the recently synthesized bismuth based Zr2(Al1−x Bi x )C MAX phases. It is revealed that the inclusion of Bi in the A site causes an increase of lattice constant a, whereas the lattice constant c decreases with increasing Bi content x up to 0.58. The c values are more influenced than the a values while Al is substituted by Bi, which implies that the c value is more dependent on the M–A bonds than the M–X bonds. The calculated band structures imply that the electrical conductivity along the c direction should be small enough compared to that in the ab plane. The low density of states (DOS) around the Fermi level indicate that Zr2(Al1−x Bi x )C should be stable in view of the electronic structure. The total DOS at the Fermi level increases almost linearly with the increase of the Bi content x between 0.25 and 0.75. The Mulliken atomic population calculations indicate that the Zr–C bonds are more covalent in Zr2BiC than that of Zr2AlC. The calculated Vickers hardness of Zr2AlC and Zr2BiC are calculated to be 5.96 and 1.94 GPa, respectively, implying that Zr2BiC is relatively soft and easily machinable compared to Zr2AlC. The calculated optical functions (dielectric constants, refractive index, extinction coefficient, absorption coefficient, loss function, reflectivity, and optical conductivity) show the dependence on the polarization directions.