Ternary Phases (Heusler) in the System Ti-Co-Sn

摘要

Some of the Heusler-phases (XY_2Z and XYZ) are known to have large homogeneity ranges which can be useful for tuning material properties. In this work, we have revised the isothermal section of the Ti-Co-Sn system at 973 K (700 °C). A total of 29 ternary compositions, mostly in the regions TiCo_(2-x)Sn for 0≤x≤ 1 and Ti_(1+y)Co_2Sn_(1-y) for 0≤y≤ 1, were prepared by arc-melting, then ball-milled and annealed. The resulting annealed powder samples were studied by applying the Rietveld method to X-ray and neutron powder diffraction data. Half-Heusler TiCoSn was not observed. The Heusler phase observed in TiCo_(2-x)Sn has compositions ranging from TiCo_(1.52)Sn to TiCo_2Sn and has the half-Heusler structure where the excess of Co is located on the semi-filled tetrahedral site Ad (3/4,3/4, 3/4) in the space group F-43m. At 1273 K (1000 °C), this solid solubility is expanded from TiCo_2Sn to TiCo with full solid solubility where Ti is gradually replacing Sn (Ti_(1+y)Co_2Sn_(1-y) for 0 ≤ y ≤1), while at 973 K (700 °C) there is a small solubility gap for 0.0 < y < 0.2.