The ternary system Yb–Cu–Mg: Isothermal section at 400 °C in the range from 0 to 67 at.% Cu

Phase relations in the ternary system Yb–Cu–Mg have been established for the isothermal section at 400 °C based on X-ray powder diffraction, metallography, SEM and EPMA techniques on about 30 alloys, which were prepared in sealed Ta-crucibles followed by a homogenisation treatment.Phase equilibria in Yb–Cu–Mg are characterized by the absence of Yb solubility in CuMg2 and of Mg solubility in YbCu2. In the compositional range 0–67 at.% Cu five ternary compounds were observed in addition to the Cu-rich ternary phase that is already reported in the literature (τ1-YbCu9Mg2). The section Cu2Mg–YbCu2 shows an extended homogeneity range for binary Cu2Mg up to 8 at.% Yb where Mg atoms are substituted by Yb. At increasing Yb-content τ2-(Yb1−xMgx)Cu2 (ordered variant of the Cu2Mg-type) forms with a homogeneity region extending from x = 0.45 to x = 0.63. A ternary phase of stoichiometry Yb2Cu2Mg (τ3) was detected, whose crystal structure is still unknown. Three compounds were found to form near the concentration line with 66.7 at.% Yb: τ4-Yb4Cu1−xMg1+x (0 ≤ x ≤ 0.25, Gd4RhIn-type), τ5-Yb23Cu7Mg4 (almost stoichiometric, with a unique structure type) and τ6-Yb2CuxMg1−x (x = 0.2, structure still unknown).Atom order in the crystal structures of YbMg2 and τ5-Yb23Cu7Mg4 (Yb23Cu7Mg4-type; P63/mmc; a = 1.1866(2); c = 2.34371(5) nm) was determined by direct methods from X-ray single crystal counter data.