First-principles study of stability, order and disorder based on an entropy descriptor in noble and ferromagnetic transition metal alloys

Binary alloys composed of ferromagnetic metals (Fe, Co, Ni) and the late noble metals (Rh, Pd, Ag, Ir, Pt, Au) have been investigated using density functional theory with the generalized gradient approximation to understand the role of magnetism in the stability and the order–disorder transition which has an impact on their physicochemical properties, their applications and their possible implementation as precursors of high-entropy alloys. The enthalpy of formation related to the stability demonstrates that all the alloys are more stable in the ferromagnetic phase than in the nonmagnetic phase. The transition from ordered to disordered phases is quantified using a descriptor which is the standard deviation of the energy spectrum of a set of small nanoalloys with random atomic configurations. The study highlights the fact that the entropy-related descriptor, which is a quantity in determining the formation of a disordered phase as a solid solution or an ordered phase is highly dependent on the atomic environment. Despite the fact that the overall variation of this descriptor is supposed to be unpredictable, there is a noticeable trend showing that the environment-dependent ferromagnetism contributes to a chemical order in alloys and nanoalloys and that this order depends on the atomic radius of the species considered. The results indicate that species with small atomic radii, such as nickel, rhodium or iridium are more likely to form solid solutions than species with larger atomic radii and with more delocalized orbitals.