WIDESPREAD INTERSTITIAL CHEMISTRY OF MN5SI3-TYPE AND RELATED PHASES -HIDDEN IMPURITIES AND OPPORTUNITIES

The many A(5)B(3) phases that exhibit hexagonal Mn5Si3-type structures by and large have a nearly unique abilities to bind diverse heteratom s Z in a preformed cavity within a chain of confacial trigonal antipri sms of A. The historical development of this chemistry as interstitial ly stabilized Nowotny phases and recent research to clarify the necess ity 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 ban d calculations are described. The special cases of hosts composed of d ivalent cations with pnictogens (As-Bi) and tetrels (Si-Pb) and their reactions with H and F are considered. The former Mn5Si3-type phase La ke up some H, but many convert at higher H or F concentrations to the orthorhombic Ca5Sb3F-type (formerly beta-Yb5Sb3) structure. Many of th e nominally valence-precise tetrelides with Cr5B3 (or other) structure s 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 de monstrate that highly significant reduction of high-temperature corros ion rates of Ti5Si3 can be secured for Ti(5)Si(3)Z, Z = C or O. The nu mber of uninvestigated Mn5Si3-type host-interstitial Z systems and the unknown ways in which various Z affect; chemical and physical propert ies in each system are very large.