Structural Acid–Base Chemistry in the Metallic State: How μ₃‑Neutralization Drives Interfaces and Helices in Ti₂₁Mn₂₅

Intermetallic phases remain a large class of compounds whose vast structural diversity is unaccounted for by chemical theory. A recent resurgence of interest in intermetallics, due to their potential in such applications as catalysis and thermoelectricity, has intensified the need for models connecting their compositions to their structures and stability. In this Article, we illustrate how the μ₃-acidity model, an extension of the acid/base concept based on the Method of Moments, offers intuitive explanations for puzzling structural progressions occurring in intermetallics formed between transition metals. Simple CsCl-type structures are frequently observed for phases with near 1:1 ratios of transition metals. However, in two compounds, TiCu and Ti₂₁Mn₂₅, structures are adopted which deviate from this norm. μ₃-Acidity analysis shows that the formation of CsCl-type phases in these exceptional systems would yield an imbalance in the acid/base strength pairing, resulting in overneutralization of the weaker partner and thus instability. Intriguing geometrical features emerge in response, which serve to improve the neutralization of the constituent elements. In both TiCu and Ti₂₁Mn₂₅, part of the structure shields weaker acids or bases from their stronger partners by enhancing homoatomic bonding in the sublattice of the weaker acid or base. In TiCu, this protection is accomplished by developing doubled layers of Ti atoms to reduce their heteroatomic contacts. In Ti₂₁Mn₂₅ the structural response is more extreme: Ti-poor TiMn₂ domains are formed to guard Mn from the Ti atoms, while the remaining Ti segregates to regions between the TiMn₂ domains. The geometrical details of this arrangement fine-tune the acid/base interactions for an even greater level of stability. The most striking of these occurs in the Ti-rich region, where a paucity of Mn neighbors leads to difficulty in achieving strong neutralization. The Ti atoms arrange themselves in helical tubes, maximizing the surface area for Ti–Mn interactions. Through these examples, we show how the μ₃-acidity model provides simple explanations for some of the beautiful structural motifs observed in intermetallic crystals. The foundation of the model in the Method of Moments makes it applicable to a variety of other contexts, including glasses, defects, and nanostructured surfaces