First-principles calculation of lattice stability of C15–M 2R and their hypothetical C15 variants (M=Al, Co, Ni; R=Ca, Ce, Nd, Y)

Abstract

When combining a stoichiometric Laves phase C15–A 2B 1 with a solid solution C15 phase(s) into a multicomponent system, a sublattice remodeling of the (A,B) 2(A,B) 1 compound is needed for the sake of database compatibility. This then requires a set of physically-grounded thermodynamic parameters for the hypothetical C15 variants (in the simplest case, A 2A, B 2B, and B 2A), in order to avoid distortion of the phase field relating to the C15 phase in the A–B phase diagram due to the sublattice remodeling. For this purpose, the present investigation employed first-principles (FP) calculations to study the lattice stability of the stable binary C15–M 2R (M=Al, Co, Ni; R=Ca, Ce, Nd, Y) and their hypothetical (unstable) C15 variants at T = 0 K . Our results demonstrated that use of the empirical parameters and energy constraint commonly used in the literature leads to a too large homogeneity range in some of the systems studied and, consequently, significant distortions of the phase diagram. In contrast, when enthalpies of formation based on FP calculations were used for the hypothetical C15 phases, such distortion of the phase diagram is minimized. The other advantage is that there is no need for re-optimization of the existing thermodynamic databases. Therefore, it is proposed that FP enthalpies of formation should be used for the thermodynamic descriptions of hypothetical C15 phases, at least when the empirical parameters fail to reproduce a reasonably accurate A–B binary phase diagram.