An ab initio study on compressibility of Al-containing MAX-phase carbides

The compressibility of Al-containing MAX carbides was investigated in details using first-principle calculations based on density functional theory. The bond stiffness and bond angle as a function of pressure were examined. The M-Al bond stiffness is about 1/3-1/2 of M-C bond stiffness. The M-C bond close to Al atoms has the highest bond stiffness in M3AlC2 and M4AlC3 phases, with the similar bond stiffness of the other two bonds in the latter. Generally, the bond stiffness of the strongest M-C bond increases with increasing VEC (Valence Electron Concentration), which also affects the bond stiffness of other bonds. Of most importance, the bulk moduli are 0.256 of the mean bond stiffness for three series. With increasing pressure, M-Al bond angle increases, but M-C bond angles decreases, which indicates that M-Al and M-C bonds shift towards basal plane and along c-axis, respectively. As a result, the compressibility becomes more difficult along c-axis than a-axis. Some abnormal phenomena in the compressibility of Al-containing Mn+1AlXn phases with VEC = 6 are attributed to the thermodynamical instability of these compounds. (c) 2013 AIP Publishing LLC.