Effect of the valence electron concentration on the bulk modulus and chemical bonding in Ta ₂AC and Zr ₂AC (A=Al, Si, and P)

We have studied the effect of the valence electron concentration, on the bulk modulus and the chemical bonding in Ta2 AC and Zr2 AC (A=Al, Si, and P) by means of ab initio calculations. Our equilibrium volume and the hexagonal ratio (ca) agree well (within 2.7% and 1.2%, respectively) with previously published experimental data for Ta2 AlC. The bulk moduli of both Ta2 AC and Zr2 AC increase as Al is substituted with Si and P by 13.1% and 20.1%, respectively. This can be understood since the substitution is associated with an increased valence electron concentration, resulting in band filling and an extensive increase in cohesion.