Widespread Interstitial Chemistry of Mn₅Si₃-Type and Related Phases. Hidden Impurities and Opportunities

The many A₅B₃ phases that exhibit hexagonal Mn₅Si₃-type structures by and large have a nearly unique abilities to bind diverse heteratoms Z in a preformed cavity within a chain of confacial trigonal antiprisms of A. The historical development of this chemistry as interstitially stabilized Nowotny phases and recent research to clarify the necessity for Z and the range of Z possible in certain hosts are described. Some hosts may bind as many as 20 different interstitial Z. Structures, stoichiometric relationships, volume effects, electronic guidelines, anomalies, Nowotny phases that require Z for stability, and a few band calculations are described. The special cases of hosts composed of divalent cations with pnictogens (As-Bi) and tetrels (Si-Pb) and their reactions with H and F are considered. The former Mn₅Si₃-type phase take up some H, but many convert at higher H or F concentrations to the orthorhombic Ca₅Sb₃F-type (formerly β-Yb₅Sb₃) structure. Many of the nominally valence-precise tetrelides with Cr₅B₃ (or other) structures react with H or F to give the new stuffed Cr₅B₃ versions with Z in a tetrahedral cavity (a Ca₅Sn₃F or La₅Pb₃O type). Some early results demonstrate that highly significant reduction of high-temperature corrosion rates of Ti₅Si₃ can be secured for Ti₅Si₃Z, Z = C or O. The number of uninvestigated Mn₅Si₃-type host-interstitial Z systems and the unknown ways in which various Z affect chemical and physical properties in each system are very large.