Coexistence of two types of short-range order in Si–Ge–Sn medium-entropy alloys

Abstract

Short-range chemical order (SRO) has been shown to play a decisive role in modulating a wide range of physical properties in medium-entropy alloys and high-entropy alloys. The enormous configurational space of these alloys implies multiple forms of SRO may exist, but such structural diversity has not been reported. Here, ab initio-based sampling reveals that Si–Ge–Sn medium-entropy alloys have two distinct forms of SRO, which carry different energies, distinct degrees of local ordering, and dissimilar electronic structures, and co-exist across a wide range of Si–Ge–Sn compositions. We rationalize the co-existence of two SROs through their virtual degeneracy of thermodynamic stability, due to a subtle balance in the change of enthalpy and configurational entropy upon transformation between the two SROs. Such co-existence of SROs suggests an inherent structural heterogeneity, a diffuse electronic structure, and a new route for band engineering in Si–Ge–Sn medium-entropy alloys. More generally, this study indicates possibility of the co-existence of multiple forms of SRO in medium- and high-entropy alloys.