Short-range order in SiSn alloy enriched by second-nearest-neighbor repulsion

Although often conceived as random solid solutions, alloys may exhibit a local short-range order (SRO) where the distribution of atoms deviates from a perfect randomness, owing to complex interactions among alloying elements. SRO can affect various properties of alloys, but understanding their exact forms, roles, and origins remains challenging from experiment alone. Here we show, through combining statistical sampling and ab initio calculations, that a strong and special SRO dominates the structure of SiSn alloy, which is a key subset of group IV alloys for mid-infrared technology. Remarkably, the SRO in SiSn is found to be reflected primarily by a strong depletion in the second Sn-Sn coordination shell. This is distinguished from the main character of the SRO in the closely related GeSn alloy, which is reflected by the depletion within the first Sn-Sn coordination shell. The unique nature of SRO in SiSn alloy is further attributed to the competition between the two unfavorable local configurations in SiSn: A Sn-Sn first-nearest neighbor through a direct bond and a Sn-Sn second-nearest neighbor through a Sn-Si-Sn motif. The large energy penalty induced by Sn-Si-Sn local structural motif is found to be responsible for the difference between SiSn and GeSn in their forms of SRO. The SRO in SiSn alloy is further demonstrated to substantially raise the direct band gap. Our finding thus constitutes a new knowledge of the origin and form of SRO and lays the foundation for understanding the complex structure of SiGeSn ternary alloy.