The isothermal section of the Tb–Co–Sb ternary system at 500 °C

The isothermal section of the phase diagram of the Tb–Co–Sb system at 500 °C was investigated mainly by X-ray powder diffraction. The binary compounds: Tb2Co17, Tb2Co7, TbCo3, TbCo2, Tb4Co3, Tb12Co7, Tb3Co, CoSb, CoSb2, CoSb3, Tb2Sb5, TbSb, Tb4Sb3 and Tb5Sb3 have been confirmed at 500 °C. No ternary compound was found in this system. The binary compound Tb2Sb5 instead of TbSb2 was found in the Tb–Sb binary system. The compound Tb2Sb5 is of Ho2Sb5-type structure (monoclinic, space group P21/m) with a = 1.31108(3), b = 0.416979(8), c = 1.46490(3) nm, β = 102.344(1)°. The homogeneity range of CoSb is about 2.0 at.% Sb at 500 °C. The maximum solid solubility of Tb in CoSb is less than 1.0 at.%. The maximum solid solubility of Tb in CoSb3 at 500 °C is small (less than 0.4 at.%), while the homogeneity ranges of the other single-phase regions are non-observable.