First-principles phase stability calculations and estimation of finite temperature effects on pseudo-binary Mg 6(Pd xNi 1−x) compounds

Abstract

The stability of pseudo-binary Mg 6(Pd xNi 1−x) compounds has been studied at T = 0 K via first-principles calculations and at 673 K by thermodynamic modelling of finite temperature effects. At 0 K, these compounds are not stable since their formation energy is above the convex hull defined by the Mg, Mg 2Ni and Mg 6Pd phases, although the energy difference is not very high. However, at 673 K, vibrational and configurational entropic effects allow the stabilisation of some of the calculated pseudo-binary Mg 6(Pd xNi 1−x) compounds. The vibrational contribution to the thermodynamic properties of the studied compounds has been calculated from Debye temperatures in the harmonic approximation. Also, the configurational entropy has been estimated taking into account the possible distribution of Pd and Ni between the several sites available in the pseudo-binary structure. The identification of intrinsic disorder and the associated energies and entropy are innovative features of this work. The phase diagram at the Mg-rich corner derived from these calculations is in fairly good agreement with recently published experimental results. In addition, the Ni for Pd substitution has been studied for the several sites available for Pd in the binary Mg 6Pd compound. The calculated preferential site occupancy is in agreement with the site occupancy factors determined in recent neutron diffraction experiments.