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

The many A 5 B 3 phases that exhibit hexagonal Mn 5 Si 3 -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 Mn 5 Si 3 -type phase take up some H, but many convert at higher H or F concentrations to the orthorhombic Ca 5 Sb 3 F-type (formerly β-Yb 5 Sb 3 ) structure. Many of the nominally valence-precise tetrelides with Cr 5 B 3 (or other) structures react with H or F to give the new stuffed Cr 5 B 3 versions with Z in a tetrahedral cavity (a Ca 5 Sn 3 F or La 5 Pb 3 O type). Some early results demonstrate that highly significant reduction of high-temperature corrosion rates of Ti 5 Si 3 can be secured for Ti 5 Si 3 Z, Z = C or O. The number of uninvestigated Mn 5 Si 3 -type host−interstitial Z systems and the unknown ways in which various Z affect chemical and physical properties in each system are very large.