Systematic study on the pressure dependence of M₂AlC phases (M=Ti,V,Cr,Zr,Nb,Mo,Hf,Ta,W)

The compressibility of the nanolaminated M₂AlC phases (space group P6₃∕mmc) with M=Ti,Zr,Hf [valence electron concentration (VEC) of the M element of 4], V, Nb, Ta (VEC=5), and Cr, Mo, W (VEC=6) is systematically studied in the pressure range from 0 to ∼70 GPa using ab initio calculations. These phases are characterized by interleaving of MC and Al layers. For VEC=4, we observe a larger compressibility along the c axis as compared to the a axis. As the VEC is increased to 5, the compressibility in the c direction decreases and becomes comparable to the compressibility in the a direction, whereas at VEC=6, the compressibility in the a direction is larger than that along the c direction. These results are consistent with recent experimental work. Based on a systematic study of the VEC-induced changes in bond stiffness, bond angle, and bond energy, we conclude that the geometric alteration of the bonding configuration in combination with the increase in M–C bond stiffness is mainly responsible for the observed compressibility change.