Phase Diagram of the BaO‐CuO Binary System

The phase diagram of the BaO‐CuO binary system has been investigated in air and in a mixed gas of Ar + 0.21 atm of O 2 . The existence of two compounds, BaCuO 2 and Ba 2 CuO 3 , was confirmed. The phase transition of Ba 2 CuO 3 from an orthorhombic to a tetragonal phase was observed to occur at 1083 K. The lattice constants of the tetragonal Ba 2 CuO 3 phase were determined to be a = 1.2975 nm and b = 0.3992 nm. BaCuO 2 was shown to melt incongruently by a synthetic reaction at 1289 K. Furthermore, the existence of two eutectic reactions and a peritectic reaction in the present system was confirmed.