EN
The isothermal section of the phase diagram of the BaO-Tb₂O_{3+δ}-CuO system at 1173 K was constructed based on X-ray powder diffraction data. The existence of the compounds BaTbO₃ (SrZrO₃ structure type, I4/mcm, a = 0.6034(1), c = 0.8576(1) nm) and Ba₄₄Cu₄₅O₉₀ (own structure type, Im-3m, a =1.8266(3) nm) was confirmed in the binary BaO-Tb₂O₃ and BaO-CuO systems, respectively, but no ternary oxides were found. The investigation of samples with nominal composition A_{0.5}Tb_{0.5}TO₃ (A = Ca, Sr or Ba, T = Ni or Zn) showed the presence of phases from the corresponding binary systems, or unreacted reagents.