  Short-range chemical order (SRO) has been recently demonstrated to play a
decisive role in modulating a wide range of physical properties in
medium-entropy alloy (MEA) and high-entropy alloy (HEA). The enormous
configurational space of these alloys implies multiple forms of SRO are likely
to develop concurrently but such structural diversity has not been reported.
Here we show, through extensive {\em ab initio}-based sampling study, that
SiGeSn medium-entropy alloys spontaneously develop two distinct forms of SRO.
Remarkably, the two types of SROs, which carry different energies, distinct
degrees of local ordering, and dissimilar electronic structures, are found to
co-exist in a wide range of compositions of SiGeSn alloys. The co-existence of
two SROs is rationalized through their virtual degeneracy of thermodynamic
stability, due to the subtle balance in the change of enthalpy and
configurational entropy upon the transformation between the two SROs. Such
co-existence of SROs thus suggests an inherent structural heterogeneity, a
diffuse electronic structure, and a new route for band engineering in SiGeSn
MEA. More generally, our finding indicates the possible ubiquity of the
co-existence of multiple forms of SRO in a broad range of MEAs and HEAs, which
has profound implications on their diverse physical properties.
