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

Abstract

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.

Crystal structure of pentalanthanum bromotriindate, La5In3Br

“…The Mn5SÌ3-type lattice is known to possess an amazing ability to host a large number of interstitials ranging from the main group elements B, C, N, O, CI, Si, Br, I to transition metals Mn, Fe, Cu, Zn, Ag and Cd, among others [1][2][3][4][5]. These compounds, derived from the Nowotny phases [6][7][8], is exemplified by LasSmBr, which has 34 valence electrons [5].…”

“…Thus the primary metal-metal interaction is between La2 and La2, Lai and In, Lai and La2, La2 and In. The bonding pattern has been analyzed previously [2,5]. …”

A new form of Ca3P2 with a ring of Dirac nodes

“…The findings suggest that the examination of the equilibrium composition of compounds in the Mn 5 Si 3 structure type, such as other alkaline earth metal phosphides and arsenides, may be of future interest. In addition, study of the exact mechanism and extent of hydrogen interstitials in stuffed Mn 5 Si 3 -type structures, drawing upon previous work, 31,32 may provide additional insight into the composition and stability of these phases and their parent compounds.…”

“…Based on previous characterization of analogous stuffed hydride Mn 5 Si 3 -type compounds, such as Ca 5 As 3 H, 31,32 the minority phase was modeled as Ca 5 P 3 H. We attribute the presence of interstitial H to hydride impurities in the reactant alkaline earth metal or to H 2 diffusion through the Ta tubes at high temperature, both of which are common sources of contamination. 31 For the majority phase, the composition was determined by performing a series of refinements where the occupancies of the Ca sites were first found to have indistinguishable occupancies and then constrained to have the same occupancy, with isotropic thermal parameters. The Ca occupancies were then fixed at incrementally changed values, and the other parameters were allowed to freely refine.…”

“…32 The inclusion of hydrogen in Mn 5 Si 3 -type phases serves a chemically intuitive stabilizing function. 31,32 Straightforward electron counting reveals that a compound with the formula A 5 B 3 , where A is an alkaline earth metal and B is a pnictogen, has one extra valence electron per formula unit (assuming the oxidation states of +2 for A and -3 for B). The inclusion of an interstitial ion such as H − or F − yields a closed-shell, charge-balanced material.…”