The Zirconium–Tin System, with Particular Attention to the Zr₅Sn3-Zr₅Sn4 Region and Zr₄Sn

The stoichiometric phases Zr₅Sn₃ and Zr₅Sn₄ (both P6₃/mcm, a = 8.4560 (7), c = 5.779 (1) Å and a = 8.7656 (7), c = 5.937 (1) Å, respectively) are obtained in high yield from powder sintering reactions of Zr and ZrSn₂ at 1000 °C. Arc-melted samples over the same composition range give generally complex and diffuse Guinier powder patterns, indicative of continuous distributions of compositions between the maximum melting ~Zr₅Sn₃₃ (Zr₅Sn₃″) and either Zr₅Sn₃ or ~Zr₅Sn₄. The arc-melted samples do not improve significantly on annealing at 1000–1100 °C, although the line phases are recovered if the samples are first ground and pelleted. We propose a phase diagram for this region in which the limiting phase fields Zr₅Sn₃ and Zr₅Sn₄ merge at high temperature and exhibit a common melting maximum. The overall behavior doubtlessly derives from the close structural relationship between Zr₅Sn₃ and Zr₅Sn₄, the latter (Ti₅Ga₄ type) being an interstitial tin derivative of the former (Mn₅Si₃ type). The structure of a single crystal that grew from arc-melted Zr₅Sn₃ was shown to have the intermediate composition Zr₅Sn_3.18, consistent with the proposed relationships and structures (a = 8.5036 (9), c = 5.820 (1) Å, R/R_w = 1.7/2.0%). The zirconium-richest compound in this system is a line phase with a composition between Zr_{4 0}Sn and Zr_4.1Sn. The compound shows weak superstructure reflections that require a doubled cell (a = 11.252 (1) Å) and a more complex structure than the assigned A15 type. Zr₄Sn slowly decomposes to α-Zr(Sn) and Zr₅Sn₃ at 820 °C.