Impurity Stabilization of Phases with the Mn₅Si₃-Type Structure. Questions Regarding La₅Sn₃ and Zr₅Si₃

The hexagonal Mn₅Si₃-type structure reported several times for La₅Sn₃ is incorrect; the binary compound is a line phase and occurs only in the tetragonal W₅Si₃-type structure. Phases with a Mn₅Si₃-type structure and lattice dimensions comparable to the earlier reports for the binary are obtained for the single-phase compositions La₅Sn₃C_0.5 (La₁₅Ge₉e structure) or La₅Sn₃O_x, x = 0.3, 1.0. Samples with the Mn₅Si₃ structure cannot be transformed to a W₅Si₃-type product by annealing. The nonmetals, which occupy interstitial positions, presumably originated with impurities in the lanthanum in earlier studies. The same conditions apply to Pr₅Sn₃ with oxygen impurities. The compound Zr₅Si₃ in the Mn₅Si₃ structure does not require interstitial C, O, etc., for stability, as previously claimed, but is stable only above ca. 1725 °C, as reported by Kocherzhinskii et al. The compound is a line phase near 1750 °C with lattice constants (a = 7.958 (1), c = 5.563 (1) Å) that are significantly larger than any reported heretofore. Annealed samples containing carbon saturate near Zr₆Si₃C₀₅ and show noticeably different lattice constants, while those with added oxygen approach the composition Zr₅Si₃O and give much smaller parameters than the binary, a = 7.9200 (6), c = 5.5502 (7) Å. Powder pattern intensity data for the latter support the binding of carbon and oxygen in the usual interstitial position in the Mn₅Si₃ structure.